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THE 

DEVELOPMENT   OF  THE   HUMAN    BODY 


MCMURRICH 


MORRIS'S  ANATOMY 

■FOURTH  EDITION 
UNDER   AMERICAN   EDITORSHIP 

Re-written,  Revised,  Iinp roved,  ivith  Many  New  Illustratiotis 

EDITED    BY 

HENRY  MORRIS,  F.R.C.S. 

Consulting   Surgeon   to,    and  formerly  Lecturer  on    Surgery  and  Anatomy  at 

Middlesex  Hospital,  London,  and  Examiner  iii  Anatomy, 

University  of  Durham,  etc. 

AND 

J.  PLAYFAIR  McMURRICH,  A.M.,  Ph.D. 

Professor  of  Anatomy  in  the  University  of  Toronto  ;  fortnerly  Professor  of 
Anatomy,  University  of  Michigan. 

Among  the  American  contributors  will  be  noted  :  J.  Playfair  McMur- 
rich,  R.  J.  Terry,  Irving  Hardesty,  G.  Carl  Huber,  Abram  T.  Kerr, 
Charles  R.  Bardeen  and  Florence  R.  Sabin.  Henry  Morris,  R. 
Marcus  Gunn  and  W.  H.  A.  Jacobson  head  the  English  contributors. 

"The  ever-growing  popularity  of  the  book  with  teachers  and  students  is  an 
index  of  its  value,  and  it  may  safely  be  recommended  to  all  interested." — From 
The  Medical  Record,  New  York. 

The  text  has  been  completely  revised.  Very  especial  attention,  in  this  new 
edition,  has  been  paid  to  the  illustrations,  with  the  result  that  the  teaching  value 
of  the  book  has  been  very  materially  increased. 

It  contains  many  features  of  special  advantage  to  students.  It  is  modern, 
up  to  date  in  every  respect.  It  has  been  carefully  revised,  and  in  many  parts 
rewritten,  and  includes  many  new  illustrations. 

Containing  about  1024  Illustrations,  of  which  many  are  in  Colors. 
One  Handsome  Octavo  Volume.  Thumb  Index.  Cloth,  $6.00. 
Sheep  or  Half  Morocco,  $7.00,  net.  Or  in  Five  Parts,  as  follows, 
each  part  sold  separately  : 

PART      I. — Morphogenesis.     Osteology.     Articulation.      Index,    ^f.50 
PART    II. — Muscles.     Organs  of  Circulation.    Lymphatics.     Index.    |2.oo 
PART  III. — Nervous  System.     Organs  of  Special  Sense.     Index.    I1.50 
PART  IV. — Organs   of  Digestion;    of   Voice  and  Respiration.      Urinary   and 

Reproductive   Organs.     Ductless    Glands.      Skin    and   Mammary 

Glands.     Index.     ;^i.5o 
PART    V. — Surgical  and  Topographical  Anatomy.     Index.     #1.00 

49*  Illustrated  Circular  upon  request. 


THE 

DEVELOPMENT    OF   THE 
HUMAN   BODY 

A  MANUAL  OF  HUMAN  EMBRYOLOGY 


J.   PLAYFAIR  McMURRICH,  A.M.,  Ph.D. 

PROFESSOR   OF    ANATOMY    IN   THE  UNIVERSITY    OF   TORONTO 
FORMERLY   PROFESSOR    OF   ANATOMY    IN   THE   UNIVERSITY   OF    MICHIGAN 


THIRD  EDITION,  REVISED  AND  ENLARGED 


With    Two  Hundred  atid  Seventy-seveti  Illustrations 


PHILADELPHIA 

P.  BLAKISTON'S  SON  &  CO. 

IOI2  WALNUT  STREET 
1907 


Copyright,  1907,  by  P.  Blakiston's  Son  &  Co. 


Press  of 

The  New  Era  printing  company 

Lancaster,  Pa. 


PREFACE  TO  THE  THIRD   EDITION. 


The  increasing  interest  in  human  and  mammalian  em- 
bryology which  has  characterized  the  ]ast  few  years  has 
resulted  in  many  additions  to  our  knowledge  of  these 
branches  of  science,  and  has  necessitated  not  a  few  correc- 
tions of  ideas  formerly  held.  In  this  third  edition  of  this 
book  the  attempt  has  been  made  to  incorporate  the  results 
of  all  important  recent  contributions  upon  the  topics  dis- 
cussed, and,  at  the  same  time,  to  avoid  any  considerable 
increase  in  the  bulk  of  the  volume.  Several  chapters  have, 
therefore,  been  almost  entirely  recast,  and  the  subject  matter 
has  been  thoroughly  revised  throughout,  so  that  it  is  hoped 
that  the  book  forms  an  accurate  statement  of  our  present 
knowledge  of  the  development  of  the  human  body. 

In  addition  to  the  works  mentioned  in  the  preface  to 
the  first  edition  as  of  special  value  to  the  student  of  Em- 
bryology, mention  should  be  made  of  the  Handbuch  der 
vergleichenden  und  experimentellen  Entzvicklungslehre  der 
Wirheltiere  edited  by  Professor  Oscar  Hertwig. 

University  of  Toronto, 
September  7,  1907. 


PREFACE  TO  THE   FIRST   EDITION. 


The  assimilation  of  the  enormous  mass  of  facts  which 
constitute  what  is  usually  known  as  descriptive  anatomy- 
has  always  been  a  difficult  task  for  the  student.  Part  of 
the  difficulty  has  been  due  to  a  lack  of  information  re- 
garding the  causes  which  have  determined  the  structure 
and  relations  of  the  parts  of  the  body,  for  without  some 
knowledge  of  the  ivhy  things  are  so,  the  facts  of  anatomy 
stand  as  so  many  isolated  items,  while  with  such  knowl- 
edge they  become  bound  together  to  a  continuous  whole 
and  their  study  assumes  the  dignity  of  a  science. 

The  great  key  to  the  significance  of  the  structure  and 
relations  of  organs  is  their  development,  recognizing  by 
that  term  the  historical  as  well  as  the  individual  develop- 
ment, and  the  following  pages  constitute  an  attempt  to 
present  a  concise  statement  of  the  development  of  the 
human  body  and  a  foundation  for  the  proper  understand- 
ing of  the  facts  of  anatomy.  Naturally,  the  individual 
development  claims  the  major  share  of  attention,  since  its 
processes  are  the  more  immediate  forces  at  work  in  de- 
termining the  conditions  in  the  adult,  but  where  the  em- 
bryological  record  fails  to  afford  the  required  data,  whether 
from  its  actual  imperfection  or  from  the  incompleteness 
of  our  knowledge  concerning  it,  recourse  has  been  had  to 
the  facts  of  comparative  anatomy  as  affording  indications 
of  the  historical  development  or  evolution  of  the  parts  under 
consideration. 

It  has  not  seemed  feasible  to  include  in  the  book  a  com- 
plete list  of  the  authorities  consulted  in  its  preparation. 


Vlll  PREFACE    TO    THE    FIRST    EDITION. 

The  short  bibhographies  appended  to  each  chapter  make 
no  pretensions  to  completeness,  but  are  merely  indications 
of  some  of  the  more  important  works,  especially  those  of 
recent  date,  which  consider  the  questions  discussed.  For 
a  very  full  bibliography  of  all  works  treating  of  human 
embryology  up  to  1893  reference  may  be  made  to  Minot's 
Bibliography  of  Vertebrate  Embryology,  published  in  the 
"  Memoirs  of  the  Boston  Society  of  Natural  History," 
volume  iv,  1893.  It  is  fitting,  however,  to  acknowledge 
an  especial  indebtedness,  shared  by  all  writers  on  human 
embryology,  to  the  classic  papers  of  His,  chief  among 
which  is  his  Anatomic  mcnschlicher  Enibryonen,  and  grate- 
ful acknowledgments  are  also  due  to  the  admirable  text- 
books of  Minot,  O.  Hertwig,  and  Kollmann. 

Anatomical  Laboratory, 
University  of  Michigan. 
October  i,  1902. 


CONTENTS. 


Introduction    i 

PART  I.— GENERAL  DEVELOPMENT. 

CHAPTER    I. 

The  Spennatozoon  and  Spermatogenesis ;  the  Ovum  and 

Its  Maturation  and  Fertilization 1 1 

CHAPTER    11. 

The  Segmentation  of  the  Ovum  and  the  Formation  of  the 

Germ  Layers 39 

CHAPTER    HI. 
The  Development  of  the  External  Form  of  the  Human 

Embryo 67 

CHAPTER    IV. 
The    Medullary    Groove,    Notochord,    and    Mesodermic 

Somites 96 

CHAPTER    V. 
The  Yolk-stalk,  Belly-stalk,  and  Fetal  Membranes 112 

PART  II.— ORGANOGENY. 

CHAPTER    VI. 
The  Development  of  the  Integumentary  System 147 

CHAPTER    VII. 
The  Development  of  the  Connective  Tissues  and  Skeleton.    160 

CHAPTER    VIII. 
The  Development  of  the  Muscular  System 203 


X  CONTENTS. 

CHAPTER    IX. 

The  Development  of  the  Circulatory  and  Lymphatic  Sys- 
tems      233 

CHAPTER    X. 
The  Development  of  the  Digestive  Tract  and  Glands ....   296 

CHAPTER    XI. 

The  Development  of  the  Pericardium,  the  Pleuro-periton- 

eum,  and  the  Diaphragm 335 

CHAPTER    XII. 
The  Development  of  the  Organs  of  Respiration 352 

CHAPTER    XIII. 
The  Development  of  the  Urinogenital  System 360 

CHAPTER    XIV. 
The  Suprarenal  System  of  Organs 392 

CHAPTER    XV. 
The  Development  of  the  Nervous  System. 400 

CHAPTER    XVI. 
The  Development  of  the  Organs  of  Special  Sense 456 

CHAPTER    XVII. 

Post-natal  Development 503 

Index    521 


THE   DEVELOPMENT 


OF  THE 


HUMAN   BODY. 


INTRODUCTION. 

Nearly  seventy  years  ago  (1839)  one  of  the  fundamental 
principles  of  biology  was  established  by  Schleiden  and 
Schwann  as  the  cell  theory.  According  to  this,  all  organ- 
isms are  composed  of  one  or  more  structural  units  termed 
cells,  each  of  which,  in  multicellular  organisms,  maintains 
an  individual  existence  and  yet  contributes  with  its  fellows 
to  the  general  existence  of  the  individual.  Viewed  in  the 
light  of  this  theory,  the  human  body  is  a  community,  an 
aggreg'ate  of  many  individual  units,  each  of  which  leads  to 
a  certain  extent  an  independent  existence  and  yet  both 
contributes  to  and  shares  in  the  general  welfare  of  the 
community. 

To  the  founders  of  the  theory  the  structural  units  were 
vesicles  with  definite  walls,  and  little  attention  was  paid 
to  their  contents.  Hence  the  use  of  the  term  "cell"  in 
connection  with  them.  Long  before  the  establishment  of 
the  cell  theory,  however,  the  existence  of  org^anisms  com- 
posed of  a  gelatinous  substance  showing  no  indications  of 
a  definite  limiting  membrane  had  been  noted,  and  in  1835 
a  French  naturalist,  Dujardin,  had  described  the  gelatinous 


2  INTRODUCTION. 

material  of  which  certain  marine  organisms  (Rhizopoda) 
were  composed,  terming  it  sarcode  and  maintaining  it  to 
be  the  material  substratum  which  conditioned  the  various 
vital  phenomena  exhibited  by  the  organisms.  Later,  in 
1846,  a  botanist,  von  Mohl,  observed  that  living  plant  cells 
contained  a  similar  substance,  upon  which  he  believed  the 
existence  of  the  cell  as  a  vital  structure  was  dependent,  and 
he  bestowed  upon  this  substance  the  name  protoplasm,  by 
which  it  is  now  universally  known. 

By  these  discoveries  the  importance  originally  attributed 
to  the  cell- wall  was  greatly  lessened,  and  in  1864  Max 
Schultze  reformulated  the  cell  theory,  defining  the  cell  as  a 
mass  of  protoplasm,  the  presence  or  absence  of  a  limiting 
membrane  or  cell-wall  being  immaterial.  At  the  same  time 
the  spontaneous  origination  of  cells  from  an  undifferen- 
tiated matrix,  believed  to  occur  by  the  older  authors,  was 
shown  to  have  no  existence,  every  cell  originating  by  the 
division  of  a  preexisting  cell,  a  fact  concisely  expressed  in 
the  aphorism  of  Virchow — oinnis  cdliila  a  cclhild. 

Interpreted  in  the  light  of  these  results,  the  human 
body  is  an  aggregate  of  myriads  of  cells,* — i.  e.,  of  masses 
of  protoplasm,  each  of  which  owes  its  origin  to  the  division 
of  a  preexistent  cell  and  all  of  which  may  be  traced  back 
to  a  single  parent  cell — a  fertilized  ovum.  All  these  cells 
are  not  alike,  however,  but  just  as  in  a  social  community  one 
group  of  individuals  devotes  itself  to  the  performance  of 
one  of  the  duties  requisite  to  the  well-being  of  the  commu- 
nity and  another  group  devotes  itself  to  the  performance  of 
another  duty,  so  too,  in  the  body,  one  group  of  cells  takes 
upon  itself  one  special  function  and  another  another.  There 
is,  in  other  words,  in  the  cell-community  a  physiological 

*  It  has  been  estimated  that  the  number  of  cells  entering  into  the 
composition  of  the  body  of  an  adult  human  being  is  about  twenty-six 
million  five  Inmdred  thousand  millions ! 


INTRODUCTION. 


division  of  labor.  Indeed,  the  comparison  of  the  cell- 
community  to  the  social  community  may  be  carried  still 
further,  for  just  as  gradations  of  individuality  may  be  rec- 
ognized in  the  individual,  the  municipality,  the  state,  and  the 
republic,  so  too  in  the  cell-community  there  are  cells ;  tissues, 
each  of  which  is  an  aggregate  of  similar  cells;  or^an^-^  which 
are  aggregates  of  tissues,  one,  however,  predominating  and 
determining  the  character  of  the  organ ;  and  systems,  which 
are  aggregates  of  organs  having  correlated  functions. 

It  is  the  province  of  embry- 
ology to  study  the  mode  of  di- 
vision of  the  fertilized  ovum 
and  the  progressive  differenti- 
ation of  the  resulting  cells  to 
form  the  tissues,  organs,  and 
systems.  But  before  consider- 
ing these  phenomena  as  seen 
in  the  human  body  it  will  be 
well  to  get  some  general  idea  of 
the  structure  of  an  animal  cell. 

This  (Fig.  i),  as  has  been  already  stated,  is  a  mass  of 
protoplasm,  a  substance  which  in  the  living  condition  is  a 
viscous  fluid  resembling  in  many  of  its  peculiarities  egg- 
albumen,  and  like  this  being  coagulated  when  heated  or 
when  exposed  to  the  action  of  various  chemical  reagents. 
As  to  the  structure  of  living  protoplasm  little  is  yet  known, 
since  the  application  of  the  reagents  necessary  for  its  accu- 
rate study  and  analysis  results  in  its  disintegration  or  coagu- 
lation. But  even  in  the  living  cell  it  can  be  seen  that  the 
protoplasm  is  not  a  simple  homogeneous  substance.  What 
is  termed  a  nucleus  is  usually  clearly  discernible  as  a  more 
or  less  spherical  body  of  a  greater  refractive  index  than  the 
surrounding  protoplasm,  and  since  this  is  a  permanent  organ 
of  the  cell  it  is  convenient  to  distinguish  the  surrounding 


Fig.  I. — Ovum  of  New-born 
Child  with  Follicle- 
cells. —  (Mcrtens. ) 


4  INTRODUCTION. 

protoplasm  as  the  cytoplasm  from  the  miclear  protoplasm  or 
karyoplasiii. 

The  study  of  protoplasm  coagulated  by  reagents  seems 
to  indicate  that  it  is  a  mixture  of  substances  rather  than 
a  simple  chemical  compound.  Both  the  cytoplasm  and  the 
karyoplasm  consist  of  a  more  solid  substance,  the  retic- 
iiluni,  which  forms  a  network  or  felt-work,  in  the  inter- 
stices of  which  is  a  more  fluid  material,  the  cnchyleiua* 
The  karyoplasm,  in  addition,  has  scattered  along  the  fibers 
of  its  reticulum  a  peculiar  material  termed  chromatin  and 
usually  contains  embedded  in  its  substance  one  or  more 
spherical  bodies  termed  nucleoli,  which  may  be  simply  larger 
masses  of  chromatin  or  bodies  of  special  chemical  compo- 
sition. And,  finally,  in  all  actively  growing  cells  there  is 
differentiated  in  the  cytoplasm  a  peculiar  body  known  as  the 
archoplasm  sphere,  in  the  center  of  which  there  is  usually  a 
minute  spherical  body  known  as  the  centrosome. 

It  has  been  already  stated  that  new  cells  arise  by  the 
division  of  preexisting-  ones,  and  this  process  is  associated 
with  a  series  of  complicated  phenomena  which  have  great 
significance  in  connection  with  some  of  the  problems  of 
embryology.  When  such  a  cell  as  has  been  described  above 
is  about  to  divide,  the  fibers  of  the  reticulum  in  the  neigh- 
borhood of  the  archoplasm  sphere  arrange  themselves  so  as 
to  form  fibrils  radiating  in  all  directions  from  the  sphere 
as  a  center,  and  the  archoplasm  with  its  contained  centro- 
some gradually  elongates  and  finally  divides,  each  portion 
retaining  its  share  of  the  radiating  fibrils,  so  that  two  asters, 


*  It  has  been  observed  that  certain  coagulable  substances  and  gelatin, 
when  subjected  to  the  reagents  usually  employed  for  "fixing"  proto- 
plasm, present  a  structure  similar  to  that  of  protoplasm,  and  it  has  been 
held  that  protoplasm  in  tlie  uncoagulated  condition  is,  like  these  sub- 
stances, a  more  or  less  homogeneous  material.  On  the  other  hand, 
Piiitschli  maintains  that  living  protoplasm  has  a  foam-structure  and  is, 
in  other  words,  an  emulsion. 


INTRODUCTION. 


5 


as  the  aggregate  of  centrosome,  sphere  and  fibrils  is  termed, 
are  now  to  be  found  in  the  cytoplasm  (Fig.  2,  A).  Grad- 
ually the  two  asters  separate  from  one  another  and  even- 
tually come  to  rest  at  opposite  sides  of  the  nucleus  (Fig.  2, 
C).     In  this  structure  important  changes  have  been  taking 


Fig.  2. — Diagrams  Illustrating  the  Prophases  of  Mitosis. — 
(Adapted  from  E.  B.   IVilson.) 

place  in  the  mean  time.  The  chromatin,  originally  scattered 
irregularly  along  the  reticulum,  has  gradually  aggregated  to 
form  a  continuous  thread  (Fig.  2,  A),  and  later  this  thread 
breaks  up  into  a  definite  number  of  pieces  termed  chromo- 
somes (Fig.  2,  B),  the  number  of  these  being  practically 


O  INTRODUCTION. 

constant  for  each  species  of  animal.  Thus,  in  man  (Dues- 
berg),  the  mouse,  the  salamander,  and  the  trout  the  number 
of  chromosomes  is  twenty- four;  in  the  ox  and  guinea-pig 
it  is  sixteen;  while  in  one  of  the  round-worms  (Ascaris) 
the  number  may  be  as  small  as  four,  or  even  two.  It  is  to 
be  noted  that  the  number  is  always  an  even  one. 

As  soon  as  the  asters  have  taken  up  their  position  on 
opposite  sides  of  the  nucleus,  the  nuclear  reticulum  begins 
to  be  converted  into  a  spindle-shaped  bundle  of  fibrils  which 
associate  themselves  with  the  astral  rays  and  have  lying 
scattered  among  them  the  chromosomes  (Fig.  2,  C).  To 
the  figure  so  formed  the  term  amphiastcr  is  applied,  and 
soon  after  its  formation  the  chromosomes  arrange  them- 
selves in  a  circle  or  plane  at  the  equator  of  the  spindle  (Fig. 
2,  D)  and  the  stages  preparatory  to  the  actual  division,  the 
prophases,  are  completed. 

The  next  stage,  the  metaphase  (Fig.  3,  A),  consists  of 
the  division,  usually  longitudinally,  of  each  chromosome, 
so  that  the  cell  now  contains  twice  as  many  chromosomes 
as  it  did  previously.  As  soon  as  this  division  is  completed 
the  anaphases  are  inaugurated  by  the  halves  of  each  chro- 
mosome separating  from  one  another  and  approaching  one 
of  the  asters  (Fig.  3,  B),  and  a  group  of  chromosomes,  con- 
taining half  of  the  total  number  formed  in  the  metaphase, 
comes  to  lie  in  close  proximity  to  each  archoplasm  sphere 
(Fig.  3,  C).  The  spindle  and  astral  fibers  gradually  resolve 
themselves  again  into  the  reticulum  and  the  chromosomes  of 
each  group  become  irregular  in  shape  and  gradually  spread 
out  upon  the  nuclear  reticulum  so  that  two  nuclei,  each  sim- 
ilar to  the  one  from  which  the  process  started,  are  formed 
(Fig.  3,  D).  Before  all  these  changes  are  accomplished, 
however,  a  constriction  makes  its  appearance  at  the  surface 
of. the  cytoplasm  (Fig.  3,  C)  and,  gradually  deepening, 
divides  the  cytoplasm  in  a  plane  passing  through  the  equa- 


INTRODUCTION. 


7 


tor  of  the  amphiaster  and  gives  rise  to  two  separate  cells 

(Fig.  3,  D). 

This  complicated  process,  which  is  known  as  karyokinesis 
or  mitosis,  is  the  one  usually  observed  in  dividing  cells,  but 


Fig.  3. — Diagrams  Illustrating  the  ]\Ietaphase  and  Anaphases 
OF  Mitosis. —  (Adapted  from  E.  B.   Wilson.) 

occasionally  a  cell  divides  by  the  nucleus  becoming  con- 
stricted and  dividing  into  two  parts  without  any  develop- 
ment of  chromosomes,  spindle,  etc.,  the  division  of  the  cell 
following  that  of  the  nucleus.  This  amitotic  method  of 
division  is,  however,  rare,  and  in  many  cases,  though  not 
always,  its  occurrence  seems  to  be  associated  with  an  impair- 


b  INTRODUCTION. 

ment  of  the  reproductive  activities  of  the  cells.  In  actively 
reproducing  cells  the  mitotic  method  of  division  may  be 
regarded  as  the  rule. 

Since  the  process  of  development  consists  of  the  multi- 
plication of  a  single  original  cell  and  the  differentiation 
of  the  cell  aggregate  so  formed,  it  follows  that  the  starting- 
point  of  each  line  of  individual  development  is  to  be  found 
in  a  cell  which  forms  part  of  an  individual  of  the  preceding- 
generation.  In  other  words,  each  individual  represents  one 
generation  i)i  esse  and  the  succeeding  generation  in  posse. 
This  idea  may  perhaps  be  made  clear  by  the  following  con- 
siderations. As  a  result  of  the  division  of  a  fertilized  ovum 
there  is  produced  an  aggregate  of  cells,  which,  by  the  physio- 
logical division  of  labor,  specialize  themselves  for  various 
functions.  Some  assume  the  duty  of  perpetuating  the  spe- 
cies and  are  known  as  the  sexual  or  germ  cells,  while  the 
remaining  ones  divide  among  themselves  the  various  func- 
tions necessary  for  the  maintenance  of  the  individual,  and 
may  be  termed  the  somatic  cells.  The  germ  cells  represent 
potentially  the  next  generation,  while  the  somatic  cells  con- 
stitute the  present  one.  The  idea  may  be  represented 
schematically  thus : 

First  generation 


Somatic  cells  -|-  germ  cells 

Second  generation 
Somatic   cells  -|-  germ  cells 

Third  generation 


Somatic  cells  -|-  germ  cells,   etc 

It  is  evident,  then,  while  the  somatic  cells  of  each  genera- 
tion die  at  their  appointed  time  and  are  differentiated  anew 
for  each  generation  from  the  germ  cells,  the  latter,  which 
may  be  termed  collectively  the  germ-plasm,  are  handed  on 
from  generation  to  generation  without  interruption,  and  it 


INTRODUCTION.  9 

may  be  supposed  that  this  has  been  the  case  ab  initio.  This 
is  the  doctrine  of  the  continuity  of  the  germ-plasm,  a  doc- 
trine of  fundamental  importance  on  account  of  its  bearings 
on  the  phenomena  of  heredity. 

It  is  necessary,  however,  to  fix  upon  some  Hnk  in  the 
continuous  chain  of  the  germ-plasm  as  the  starting-point 
of  the  development  of  each  individual,  and  this  link  is  the 
fertilized  ovum.  By  this  is  meant  a  germ  cell  produced  by 
the  fusion  of  two  units  of  the  germ-plasm.  In  many  of 
the  lower  forms  of  life  {e.  g.,  Hydra  and  certain  turbel- 
larian  worms)  reproduction  may  be  accomplished  by  a  di- 
vision of  the  entire  organism  into  two  parts  or  by  the 
separation  of  a  portion  of  the  body  from  the  parent  indi- 
vidual. Such  a  method  of  reproduction  is  termed  non- 
sexual.  Furthermore  in  a  number  of  forms  {e.  g.,  bees. 
Phylloxera,  water- fleas)  the  germ  cells  are  able  to  undergo 
development  without  previously  being  fertilized,  this  consti- 
tuting a  method  of  reproduction  known  as  parthenogenesis. 
But  in  all  these  cases  sexual  reproduction  also  occurs,  and 
in  all  the  more  highly  organized  animals  it  is  the  only 
method  which  normally  occurs ;  in  it  a  germ  cell  develops 
only  after  complete  fusion  with  another  germ  cell.  In  the 
simpler  forms  of  this  process  little  difference  exists  between 
the  two  combining  cells,  but  since  it  is,  as  a  rule,  of  advan- 
tage that  a  certain  amount  of  nutrition  should  be  stored 
up  in  the  germ  cells  for  the  support  of  the  developing  em- 
bryo until  it  is  able  to  secure  food  for  itself,  while  at  the 
same  time  it  is  also  advantageous  that  the  cells  which  unite 
shall  come  from  different  individuals  (cross-fertilization), 
and  hence  that  the  cells  should  retain  their  motility,  a  di- 
vision of  labor  has  resulted.  Certain  germ  cells  store  up 
more  or  less  food  yolk,  their  motility  becoming  thereby  im- 
paired, and  form  what  are  termed  the  female  cells  or  ova, 
while  others  discard  all  pretensions  of  storing  up  nutrition. 


lO  INTRODUCTION. 

are  especially  motile  and  can  seek  and  penetrate  the  inert 
ova;  these  latter  cells  constitute  the  male  cells  or  sperma- 
tozoa. In  many  animals  both  kinds  of  cells  are  produced 
by  the  same  individual,  but  in  all  the  vertebrates  (with  rare 
exceptions  in  some  of  the  lower  orders)  each  individual 
produces  only  ova  or  spermatozoa,  or,  as  it  is  generally 
stated,  the  sexes  are  distinct. 

It  is  of  importance,  then,  that  the  peculiarities  of  the 
two  forms  of  germ  cells,  as  they  occur  in  the  human  species, 
should  be  considered. 

LITERATURE. 
E.  B.   Wilson  :   "  The  Cell  in  Development  and  Inheritance."     Third 

edition.     New  York,  1900. 
O.  Hertwig  :  "  Die  Zelle  und  die  Gewebe."     Jena,  1893. 


PART    I. 

GENERAL   DEVELOPMENT. 


CHAPTER   L 

THE  SPERMATOZOON  AND  SPERMATOGENESIS; 
THE  OVUM  AND  ITS  MATURATION  AND 
FERTILIZATION. 

The  Spermatozoon. — The  human  spermatozoon  (Fig. 
4)  is  a  minute  and  greatly  elongated  cell,  measuring  about 
0.05  mm.  in  length  and  consisting  of  an  anterior  broader 
portion  or  head  (k)  and  a  narrow  thread-like  tail  (/).  The 
head  measures  about  0.005  ^'^^^i-  i^^  length  and  when  viewed 
from  one  surface  (Fig.  4,  i)  has  an  oval  outline,  though 
since  it  is  somewhat  flattened  or  concave  toward  the  tip, 
it  has  a  pyriform  shape  when  seen  in  profile  (Fig.  4,  2). 
The  tail  consists  of  several  portions.  Situated  immediately 
behind  the  head  is  a  short  cylindrical  portion  measuring 
0.006  mm.  in  length  which  is  termed  the  middle-piece  or 
neck  (ill),  and  behind  this  is  the  flageUuin,  of  about  the 
same  diameter  as  the  middle-piece,  but  forming  about  four 
fifths  (0.04  mm.)  of  the  entire  length  of  the  spermatozoon. 
The  axis  of  the  flagellum  is  formed  by  a  delicate  filament 
which  projects  somewhat  beyond  the  flagellum,  forming 
what  is  termed  the  tenninal  filament  or  end-piece  {c). 

In  addition  to  these  various  parts,  the  spermatozoa  of  many 
mammalia  possess  a  head-cap  (Fig.  5,  he)  covering  the  anterior 
end  of  the  head,  and  a  spiral  membrane  wound  around  the 
flagellum.     The  presence  of  these  structures  has  not  yet  been 


12 


THE    SPERMATOZOON. 


generally  observed  in  the  human  spermatozoon,  though  several 
observers  have  claimed  the  existence  of  a  spiral  membrane  and 
the  head-cap  undoubtedly  exists  in  the  earlier  stages  of  the 
development  of  the  spermatozoon,  though  it  may  later  be  lost. 


\m 


,6 


Fig.   4. — Human    Spermatozoon. 

I,  Front  view,  2,  side  view  of  the 
head ;  c,  terminal  filament ;  k, 
head ;  /,  tail ;  m,  middle-piece. 
{After  Retzius.) 


Fig.    5. — Spermatozoon    of    Rat. 
h,  Head ;  he,  head-cap ;  ni[>,  mid- 
dle-piece;   H,    neck. —  {Jensen.) 


To  unders,tand  the  significance  of  the  various  parts  enter- 
ing into  the  composition  of  the  spermatozoon  a  study  of 
their  development  is  necessary,  and  since  the  various  proc- 
esses of  spermatogenesis  have  been  much  more  accurately 
observed  in  such  mammalia  as  the  rat  and  guinea-pig  than 
in  man,  the  description  which  follows  will  be  based  on  what 
has  been  described  as  occurring  in  these  forms.  From  what 
is  known  of  the  spermatogenesis  in  man  it  seems  certain 
that  it  closely  resembles  that  of  these  mammals  so  far  as  its 
essential  features  are  concerned. 


SPERMATOGENESIS. 


13 


Spermatogenesis. — The  spermatozoa  are  developed  from 
the  cells  which  line  the  interior  of  the  seminiferous  tubules 
of  the  testis.  The  various  stages  of  development  cannot 
all  be  seen  at  any  one  part  of  a  tubule,  but  the  formation 
of  the  spermatozoa  seems  to  pass  along  each  tubule  in  a 
wave-like  manner  and  the  appearances  presented  at  differ- 
ent points  of  the  wave  may  be  represented  diagrammatically 
as  in  Fig.  6. 

In  the  first  section  of  this  figure  four  different  genera- 


FiG.  6. — Diagram  showing  Stages  of  Spermatogenesis  as  seen  in 
Different  Sectors  of  a  Seminiferous  Tubule  of  a  Rat. 

s,  Sertoli  cell;  sc^,  spermatocyte  of  the  first  order;  sc",  spermatocyte  of 
the  second  order;  sg,  spermatogone;  sp,  spermatid;  ss,  spermato- 
zoon.—  {Modified  from  von  Lcnhossek.) 

tions  of  cells  are  represented;  above  are  mature  sperma- 
tozoa lying  in  the  lumen  of  the  tubule,  while  next  the  base- 
ment membrane  is  a  series  of  cells  from  which  a  new  gen- 
eration of  spermatozoa  is  about  to  develop.  The  cells  of 
this  series  are  of  two  kinds;  the  larger  one  (s)  will  develop 
into  a  structure  known  as  a  Sertoli  cell,  while  the  others  are 
parent  cells  of  spermatozoa  and  are  termed  spermatogonia 
(sg).  In  the  next  section  the  Sertoli  cell  is  seen  to  have 
become    considerably    enlarged,    its    cytoplasm    projecting 


14  SPERMATOGENESIS. 

toward  the  lumen  of  the  tubule,  and  in  the  third  section  the 
enlargement  has  increased  to  such  an  extent  that  the  sper- 
matogonia are  forced  away  from  the  basement  membrane, 
with  which  the  Sertoli  cell  alone  is  in  contact.  In  the  fourth 
section  the  spermatogonia  are  seen  in  process  of  division; 
one  of  the  cells  so  formed  will  persist  as  a  spermatogone, 
while  the  other  forms  what  is  termed  a  primary  sperma- 
tocyte (sc^).  The  results  of  the  division  are  seen  in  the 
last  section,  where  four  spermatogonia  are  seen  again  in 
contact  with  the  basement  membrane  and  above  them  are 
four  primary  spermatocytes.  Returning  now  to  the  first 
and  second  sections,  the  layer  of  primary  spermatocytes 
may  still  be  seen,  indications  of  an  approaching  division 
being  furnished  by  the  arrangement  of  the  chromatin  in 
those  of  the  second  section,  and  in  the  third  section  the 
division  is  seen  in  progress,  the  two  cells  which  result  from 
it  being  termed  secondary  spermatocytes  (sc").  These  cells 
almost  immediately  underg'o  division,  as  shown  in  the  fourth 
section,  each  giving  rise  to  two  spermatids  (sp),  each  of 
which  becomes  later  on  directly  transformed  into  a  sperma- 
tozoon (^-^).  From  the  primary  spermatocyte  there  have 
been  formed,  therefore,  as  the  result  of  two  mitoses,  four 
cells,  each  of  which  represents  a  spermatozoon. 

During  these  divisions  important  departures  from  the 
typical  method  of  mitosis  occur.  These  departures  have 
been  most  thoroughly  studied  in  the  lower  forms,  but  it  is 
probable  that  they  are  fundamentally  similar  in  the  mam- 
malia. It  has  already  been  pointed  out  (p.  6)  that  the 
number  of  chromosomes  which  appear  during  the  mitoses 
of  the  somatic  cells  is  characteristic  for  the  species.  In 
the  division  of  the  primary  spermatocytes  the  number  of 
chromosomes  which  appear  is  apparently  only  half  the  char- 
acteristic number,  but  in  reality  it  is  double  that  number, 
since  each  chromosome  is  really  composed  of  four  elements 


SPERMATOGENESIS. 


IS 


more  or  less  closely  united  to  form  a  tetrad.  During  the 
mitosis  each  tetrad  divides  into  two  dyads,  one  of  which 
passes  into  each  secondary  spermatocyte,  and  these  cells 
undergoing  division  without  the  usual  reconstruction  of  the 
nucleus,  each  of  the  dyads  which  they  contain  is  halved. 


Fig.   7. — ^DiAGRAM    Illustrating    the    Reduction    of    the    Chromo- 
somes  During   Spermatogenesis. 
sc^,   Spermatocyte   of  the   first  order;   sc^,  spermatocyte   of  the   second 
order;   sp,  spermatid. 

so  that  each  spermatid  receives  a  number  of  single  chromo- 
somes equal  to  half  the  number  characteristic  for  the  species. 
This  reduction  of  the  chromosomes  of  the  germ  cells  may 
be  understood  from  the  annexed  diagram  (Fig.  7),  which 
represents  the  spermatogenesis  of  a  form  whose  somatic 
cells  are  supposed  to  contain  eight  chromosomes. 


i6 


SPERMATOGENESIS. 


The  transformation  of  the  spermatids  into  spermatozoa 
takes  place  while  they  are  in  intimate  association  with  the 
Sertoli  cells,  a  number  of  them  fusing  with  the  cytoplasm 
of  an  enlarged  Sertoli  cell,  as  shown  in  Fig.  6,  s,  and  prob- 
ably receiving  nutrition  from  it.     In  each  spermatid  there 

is  present,  in  addition  to  the 
nucleus,  an  archoplasm  sphere, 
from  which  the  centrosomes 
have  migrated  so  as  to  lie  free 
in  the  cytoplasm.  The  details 
of  the  transformation  are  still 
to  a  certain  extent  under  dis- 
cussion, the  view  here  pre- 
sented being  only  one  of  the 
many  which  have  been  ad- 
vanced within  recent  years. 
On  the  fusion  of  the  sper- 
matid with  a  Sertoli  cell,  a 
delicate  filament  (Fig.  8,  /), 
the  beginning-  of  the  axial  fila- 
ment of  the  spermatozoon. 
Fig.  8.-F0UR  Stages  in  the  appears  in  its  Cytoplasm,  seem- 
Transformation    of    a    Sper-   iiipr  to  arise  from  the  centro- 

MATID       INTO       THE       SpERMATO-  ,    .    ,        . 

zooN  OF  A  Rat.  some  which  lies  at  one  end  of 

a,  Archoplasm;  c,  mass  of  chro-  it.    The  aixhoplasm  Sphere  (a) 

matin  which   is   later  absorbed ; 

/,  axial  filament;  h,  head;  he,  and    centrosome    migrate    to 

head-cap;    w/>     middle-piece.-  opposite  sides  of  the  iiucleus, 
{zwn  Lenhossck.)  ^  ^ 

which   gradually  assumes   an 

excentric  position,  and  the  archoplasm  becomes  converted 

into  the  head-cap   {he)    while  the  centrosomes,  enlarging, 

form  the  anterior  portion  or  neck  of  the  middle-piece  (mp), 

the  remainder  of  that  structure  being  formed  from  the  axial 

filament  surrounded  by  a  cytoplasmic  sheath.     As  the  axial 

filament  lengthens  the  cytoplasm  is  drawn  out  with  it  to 


THE    OVUM,  17 

form  its  sheath,  the  terminal  portion  of  the  filament  only 
projecting"  beyond  the  sheath  to  form  the  end-piece,  and  the 
cytoplasm  surrounding  the  nucleus  becomes  reduced  to  an 
exceedingly  delicate  layer,  so  that  the  head  of  the  spermato- 
zoon (h)  consists  almost  entirely  of  nuclear  substance  if 
the  head-cap  be  left  out  of  consideration. 

The  homologies  of  the  parts  of  the  spermatozoon  with 
those  of  the  spermatid  may  be  presented  in  tabular  form 
thus : 

Spermatid.  Spermatozoon. 

Nucleus.  Head. 

Archoplasm.  Head-cap. 

Centrosome.  Neck  of  middle-piece. 

i  Axial  filament. 

Cytoplasm.  <  Sheath  of  middle-piece. 

(^  Sheath  of  tail. 

The  spermatozoon  is,  then,  one  of  four  equivalent  cells, 
produced  by  two  successive  divisions  of  a  primary  spermat- 
ocyte and  containing  one  half  the  number  of  chromosomes 
characteristic  for  the  species. 

The  Ovum. — The  human  ovum  is  a  spherical  cell  meas- 
uring about  0.2  mm.  in  diameter  and  is  contained  within 
a  cavity  situated  near  or  at  the  surface  of  the  ovary  and 
termed  a  Graafian  follicle.  This  follicle  is  surrounded  by  a 
capsule  composed  of  two  layers,  an  outer  one,  the  theca 
externa,  consisting  of  fibrous  tissue  resembling  that  found 
in  the  ovarian  stroma,  and  an  inner  one,  the  theca  interna, 
composed  of  numerous  spherical  and  fusiform  cells.  Both 
the  thecse  are  richly  supplied  with  blood-vessels,  the  theca 
interna  especially  being  the  seat  of  a  very  rich  capillary 
network.  Internal  to  the  theca  interna  there  is  a  trans- 
parent, thin,  and  structureless  hyaline  nieinbrane,  within 
which  is  the  follicle  proper,  whose  wall  is  formed  by  a 
layer  of  cells  termed  the  stratum  granulosiini  (Fig.  9,  mg) 
and  inclosing  a  cavity  filled  with  an  albuminous  fluid,  the 
3 


l8  THE    OVUM, 

liquor  follicuU.  At  one  point,  nsually  on  the  surface  nearest 
the  center  of  the  ovary,  the  stratum  granulosum  is  greatly 
thickened  to  form  a  mass  of  cells,  the  discus  proligerus 
(dp),  which  projects  into  the  cavity  of  the  follicle  and  en- 
closes the  ovum  (o).     Usually  but  a  single  ovum  is  con- 


^       ( 


s 


o 


my. 


Fig.  9. — Section  through  Portion  -of  an  Ovary  of  an  Opossum 
{Didelphys  virginiana)  showing  Ova  and  Follicles  in  Various 
Stages   of  Development. 

b,  Blood-vessel;  dp,  discus  proligerus;  vig,  stratum  granulosum;  0, 
ovum ;  s,  stroma ;  //i,  theca  folliculi. 

tained  in  any  discus,  though  occasionally  two  or  even  three 
may  occur. 

The  cells  of  the  discus  proligerus  are  for  the  most  part 
more  or  less  spherical  or  ovoid  in  shape  and  are  arranged 
irregularly.  In  the  immediate  vicinity  of  the  ovum,  how- 
ever, they  are  more  columnar  in  form  and  are  arranged  in 
about  two  concentric  rows,  thus  giving  a  somewhat  radiated 


THE    OVUM.  19 

appearance  to  this  portion  of  the  discus,  which  is  termed 
the  corona  radiafa  (Fig.  10,  cr).  Immediately  within  the 
corona  is  a  transparent  membrane,  the  j:ona  pellucida  (Fig. 
10,  ^p),  about  as  thick  as  one  of  the  cell  rows  of  the  corona 
(0.02  to  0.024  mm.),  and  presenting  a  very  fine  radial  stria- 
tion  which  has  been  held  to  be  due  to  minute  pores  travers- 
er        -r^   ^  ^^-^^^.--^ffp)  „ 


k' 


'-c^    y 


'■<i 


v-^, 


""3, 


.[-'P 


^^-• 


(A^ 


ps  ---' 

Fig.    10. — Ovum   from    Ovary  of  a   Woman   Thirty  Years   of   Age. 

cr,  Corona  radiata ;  n,  nucleus ;  p,  protoplasmic  zone  of  ovum ;  ps,  peri- 

vitelline  space;  y,  yolk;  sp,  zona  pellucida. —  (Nagel.) 

ing  the  membrane  and  containing  delicate  prolongations  of 
the  cells  of  the  corona  radiata.  Within  the  zona  pellucida 
is  the  ovum  proper,  whose  cytoplasm  is  more  or  less  clearly 
differentiated  into  an  outer  more  purely  protoplasmic  por- 
tion (Fig.  10,  p)  and  an  inner  deutoplasmic  mass  (y)  which 
contains  numerous  fine  granules  of  fatty  and  albuminous 


20  -  OVULATION. 

natures.  These  granules  represent  the  food  yolk  or  deuto- 
plasm,  which  is  usually  much  more  abundant  in  the  ova  of 
other  mammals  and  forms  a  mass  of  relatively  enormous 
size  in  the  ova  of  birds  and  reptiles.  The  nucleus  (ii)  is 
situated  somewhat  excentrically  in  the  deutoplasmic  portion 
of  the  ovum  and  contains  a  single,  well-defined  nucleolus. 

A  follicle  with  the  structure  described  above  and  con- 
taining a  fully  grown  ovum  may  measure  anywhere  from 
five  to  twelve  millimeters  in  diameter,  and  is  said  to  be 
"  mature,"  having  reached  its  full  development  and  being 
ready  to  burst  and  set  free  the  ovum.  This,  however,  is 
not  yet  mature;  it  is  not  ready  for  fertilization,  but  must 
first  undergo  certain  changes  similar  to  those  through  which 
the  spermatocyte  passes,  the  so-called  ovum  at  this  stage 
being  more  properly  a  primary  oocyte.  But  before  describ- 
ing- the  phenomena  of  maturation  of  the  ovum  it  will  be 
well  to  consider  the  extrusion  of  the  ovum  and  the  changes 
which,  the  follicle  subsequently  undergoes. 

Ovulation  and  its  Relation  to  Menstruation. — As  a 
rule,  but  a  single  follicle  near  maturity  is  found  in  either 
the  one  or  the  other  ovary  at  any  given  time.  In  the  early 
stages  of  its  development  a  follicle  is  situated  somewhat 
deeply  in  the  stroma  of  the  ovary,  but  during  its  growth 
it  approaches  the  surface  and  eventually  forms  a  marked 
prominence,  only  an  exceedingly  thin  membrane  separating 
the  cavity  of  the  follicle  from  the  abdominal  cavity.  This 
thin  membrane  finally  ruptures,  and  the  liquor  folliculi, 
which  is  apparently  under  some  pressure  while  contained 
within  the  follicle,  rushes  out  through  the  rupture,  carrying 
with  it  the  ovum  surrounded  by  some  of  the  cells  of  the 
discus  proligerus. 

The  immediate  cause  of  the  bursting  of  the  follicle  is  not 
yet  clearly  understood.  It  has  been  suggested  that  a  gradual 
increase  of  the  liquor  folliculi  under  pressure  must  in  itself 


OVULATION.  21 

finally  lead  to  a  rupture,  and  it  has  also  been  pointed  out 
that  just  before  the  maturation  of  the  follicle  the  theca 
interna  undergoes  an  exceedingly  rapid  development  and 
vascularization  which  may  play  an  important  part  in  the 
phenomenon. 

Normally  the  ovum  when  expelled  from  its  follicle  is 
received  at  once  into  the  Fallopian  tube,  and  so  makes  its 
way  to  the  uterus,  in  whose  cavity  it  undergoes  its  develop- 
ment. Occasionally,  however,  this  normal  course  may  be 
interfered  with,  the  ovum  coming  to  rest  in  the  tube  and 
there  undergoing  its  development  and  producing  a  tubal 
pregnancy;  or,  ag'ain,  the  ovum  may  not  find  its  way  into 
the  Fallopian  tube,  but  may  fall  from  the  follicle  into  the 
abdominal  cavity,  where,  if  it  has  been  fertilized,  it  will 
undergo  development,  producing  an  abdominal  pregnancy; 
and,  finally,  and  still  more  rarely,  the  ovum  may  not  be  ex- 
pelled when  the  Graafian  follicle  ruptures  and  yet  may  be 
fertilized  and  undergo  its  development  within  the  follicle, 
bringing  about  what  is  termed  an  ovarian  pregnancy.  All 
these  varieties  of  extra-uterine  pregnancy  are,  of  course, 
exceedingly  serious,  since  in  none  of  them  is  the  fetus 
viable. 

It  was  long  believed  that  ovulation  was  coincident  with 
certain  periodic  changes  of  the  uterus  which  constitute  what 
is  termed  menstruation.  This  phenomenon  makes  its  ap- 
pearance at  the  time  of  puberty,  the  exact  age  at  which  it 
appears  being  determined  by  individual  and  racial  peculiari- 
ties and  by  climate  and  other  factors,  and  after  it  has  once 
appeared  it  normally  recurs  at  definite  intervals  more  or 
less  closely  corresponding  with  lunar  months  (/'.  e.,  at  inter- 
vals of  about  twenty-eight  days,  the  extremes  being  from 
twenty- four  to  thirty-four  days)  until  somewhere  in  the 
neighborhood  of  the  fortieth  or  forty-fifth  year,  when  it 
ceases. 


22  MENSTRUATION, 

The  structural  chang-es  associated  with  menstruation  con- 
sist of  a  prehminary  thickening  of  the  walls  of  the  uterus, 
its  mucous  membrane  and  the  subjacent  tissue  becoming 
highly  vascular  and  eventually  congested.  Later  the  walls 
of  the  blood-vessels  degenerate  and  permit  of  an  escape  of 
blood  here  and  there  beneath  the  mucous  membrane  which, 
in  the  areas  overlying  the  effused  blood,  undergoes  a  fatty 
degeneration  and  is  descjuamated,  allowing  of  the  formation 
of  a  blood-clot  in  the  cavity  of  the  uterus.  The  hemor- 
rhagic portion  of  the  process  lasts  usually  from  three  to  five 
days;  at  its  close  a  regeneration  of  the  lost  portions  of  the 
mucous  membrane  begins,  and  when  this  is  completed  a 
resting  period  ensues  which  persists  until  near  the  time  of  a 
new  menstrual  period. 

The  local  structural  changes  of  the  uterus  are  associated 
with  decided  constitutional  disturbances.  The  pulse,  blood- 
pressure,  temperature,  muscular  power,  and  lung  capacity 
are  in  general  somewhat  increased  before  menstruation  and 
sink  immediately  before  or  at  the  time  when  the  hemorrhage 
in  the  uterus  begins;  immediately  before  the  menstrual 
period  there  is  also  a  diminished  destruction  of  the  nitroge- 
nous materials  of  the  body,  as  shown  by  the  amount  of 
nitrogen  excreted  being  less  than  at  other  times. 

These  general  changes  may  well  affect  the  ovary  as  well 
as  other  portions  of  the  body  and  so  contribute  to  a  coin- 
cidence of  menstruation  and  ovulation.  And,  indeed,  there 
seems  little  question  but  that  the  coincidence  is  of  frequent 
or  even  usual  occurrence.  The  appearance  of  menstruation 
indicates,  as  a  rule,  the  beginning  of  fertility,  and  sterility 
ensues  at  the  time  of  the  final  cessation  of  the  menses. 
Furthermore,  menstruation  ceases  when  pregnancy  super- 
venes, and  the  cessation  persists  not  only  until  parturition, 
but  so  long  as  the  child  remains  unweaned,  and  as  a  rule 
ovulatiDu  is  also  in  abe3^ince  during  the  same  period.     Ex- 


RELATION    OF    OVULATION    TO    MENSTRUATION.  23 

ceptions,  however,  have  been  observed  which  show  that  the 
coincidence  of  the  two  phenomena  is  not  invariable,  preg- 
nancy, for  example,  having  occurred  in  young  girls  who  had 
not  yet  menstruated,  and  in  forty-two  operated  cases  in 
which  the  ovaries  and  uterus  had  been  removed  after  men- 
struation, twelve  showed  no  signs  of  ovulation  as  deter- 
mined by  the  presence  of  recently  ruptured  follicles  in  the 
ovaries  (Leopold  and  MironofT),  while  in  another  set  of 
fifty-four  cases  ovulation  appeared  to  have  coincided  with 
menstruation  in  thirty-nine  instances.   . 

From  the  evidence  at  present  at  our  disposal  it  may  be 
stated  that  in  the  human  species  while  ovulation  generally 
coincides  with  menstruation,  yet  the  two  phenomena  may, 
and  not  infrequently  do,  occur  independently  of  one  another. 

In  the  lower  mammals  ovulation  is,  as  a  rule,  directly  asso- 
ciated with  a  condition  known  as  oestrus  or  "heat,"  this  being 
preceded  by  certain  phenomena  constituting  what  is  termed 
the  prooestrum  and  corresponding  essentially  to  menstruation. 
In  the  majority  of  species  one  or  more  oestrous  periods  occur 
during  the  year  in  fertile  females,  and  each  of  these  is  pre- 
ceded by  a  prorestrum.  But  the  procestrous  phenomena  do 
not  in  all  forms  invariably  lead  to  an  oestrum,  since,  although 
female  monkeys  menstruate  regularly  throughout  the  year,  yet 
they  have  only  one  annual  oestrous  period  (Heape). 

The  immediate  causes  of  the  procestrous  and  oestrous  phe- 
nomena and  of  their  periodic  occurrence  are  as  yet  obscure. 
Animals,  however,  from  which  the  ovaries  have  been  completely 
removed  do  not  exhibit  the  phenomena,  and  it  has  recently 
been  found  that  in  such  animals  an  oestrous  state,  or  at  least 
a  transient  condition  resembling  oestrus  may  be  produced  by 
the  injection  of  an  extract  of  ovaries  taken  from  animals  in  a 
procestrous  or  oestrous  state.  On  the  basis  of  this  it  has  been 
suggested  that  the  procestrous  and  oestrous  phenomena  are  the 
results  of  an  internal  secretion  produced  by  the  ovaries  (Mar- 
shall and  Jolly). 

The  Corpus  Luteum. — With  the  setting  free  of  the 
ovum  the  usefulness  of  the  Graafian  follicle  is  at  an  end, 
and  it  begins  at  once  to  undergo  retrogressive  changes  which 


24 


CORPUS    LUTEUM. 


result  primarily  in  the  formation  of  a  structure  known  as 
the  corpus  luteuui  (Fig.  ii).  On  the  rupture  of  the  follicle 
a  considerable  portion  of  the  stratum  granulosum  remains 
in  place,  and  usually  there  is  an  effusion  of  a  greater  or  less 
amount  of  blood  from  the  vessels  of  the  theca  interna  into 

the  follicular  cavity.  The 
split  in  the  wall  through 
which  the  ovum  escaped 
soon  closes  over  and  the 
cavity  becomes  filled  with 
cells  separated  into  groups 
by  trabeculse  of  connective 
tissue  containing  blood- 
vessels (Fig.  12).  These 
cells  contain  a  consider- 
able amount  of  a  peculiar 
yellow  pigment  known  as 
lutein,  the  color  imparted 
to  the  follicle  by  this  sub- 
stance having  suggested  the  name  corpus  luteum  which  is 
now  applied  to  it. 

In  later  stages  there  is  a  gradual  increase  in  the  amount 
of  connective  tissue  present  and  a  corresponding  diminu- 
tion of  the  lutein  cells,  the  corpus  luteum  gradually  losing 
its  yellow  color  and  becoming  converted  into  a  whitish, 
fibrous,  scar-like  body,  the  corpus  albicans,  which  may  event- 
ually almost  completely  disappear.  These  various  changes 
occur  in  every  ruptured  follicle,  whether  or  not  the  ovum 
which  was  contained  in  it  be  fertilized.  But  the  rapidity 
with  which  the  various  stages  of  retrogression  ensue  differs 
greatly  according  to  whether  pregnancy  occurs  or  not,  and 
it  is  customary  to  distinguish  the  corpora  lutea  which  are 
associated  with  pregnancy  as  corpora  lufca  z'cra  from  those 
whose  ova  fail  to  be  fertilized  and  which  form  corpora  lutea 


Fig.  II. — Ovary  of  a  Woman  Nine- 
teen Years  of  Age,  Eight  Days 
AFTER  Menstruation. 

d,  Blood-clot;  /,  Graafian  follicle;  th, 
theca. —  {Kollmann.) 


THE    CORPUS    LUTEUM. 


25 


Spuria.  In  the  latter  the  retrogression  of  the  follicle  is 
completed  usually  in  about  five  or  six  weeks,  while  the  cor- 
pora vera  persist  throughout  the  entire  duration  of  the  preg- 
nancy and  complete  their  retrogression  after  the  birth  of 
the  child. 

Two  very  different  views  are  held  as  to  the  origin  of  the 


-fe 


Fig.  12. — Section  through  the  Corpus  Luteum  of  a  Rabbit, 
Seventy  Hours  post  coitum. 

The  cavity  of  the  follicle  is  almost  completely  filled  with  lutein  cells 
among  which  is  a  certain  amount  of  connective  tissue,  g,  Blood- 
vessels;   ke,   ovarial   epithelium. —  (Sobotta.) 

lutein  cells.  According  to  one,  which  may  be  termed  von 
Baer's  view,  the  cells  of  the  stratum  granulosum  remain- 
ing in  the  follicle  rapidly  undergo  degeneration  and  com- 
pletely disappear,  and  the  lutein  cells  and  connective-tissue 

4 


26  CORPUS    LUTEUM. 

trabecul?e  are  formed  entirely  from  the  cells  of  the  theca 
interna,  which  increase  rapidly  both  in  size  and  number. 
The  other  view  was  first  advanced  by  Bischoff  and  may  be 
known  by  his  name.  It  is  to  the  effect  that  the  granulosa 
cells  do  not  disintegrate,  but,  on  the  contrary,  increase  rap- 
idly in  number  and  become  converted  into  the  lutein  cells, 
only  the  connective  tissue  and  the  blood-vessels  being  derived 
from  the  theca  interna. 

Which  of  these  two  views  is  correct  is  at  present  uncer- 
tain. The  majority  of  those  who  have  within  recent  years 
studied  the  formation  of  the  human  corpus  luteum  have 
expressed  themselves  in  favor  of  von  Baer's  theory.  So- 
botta  has,  however,  made  a  thorough  study  of  the  phenom- 
ena in  a  perfect  series  of  mice  ovaries  and  has  demonstrated 
that  in  that  form  the  lutein  cells  are  derived  from  the  granu- 
losa cells.  It  would  be  strange  if  the  lutein  cells  had  a 
different  origin  in  two  different  mammals,  and  the  observa- 
tions on  mice  are  so  thorough  that  one  is  tempted  to  regard 
different  results  as  being  due  to  imperfections  in  the  series 
of  ovaries  studied,  important  steps  in  the  development  of 
the  corpora  lutea  being  thus  overlooked.  This  temptation 
is,  moreover,  greatly  increased  by  the  fact  that  Sobotta's 
observations  have  been  confirmed  in  the  cases  of  several 
other  animals,  such,  for  instance,  as  the  rabbit  (Sobotta, 
Honore,  Cohh),  certain  bats  (van  der  Stricht),  the  sheep 
(Marshall),  the  marsupial  dasyurus  (Sandes),  the  spermo- 
phile  (Volker),  and  the  guinea-pig  (Sobotta).  The  weight 
of  evidence  is  at  the  present  time  strongly  in  favor  of 
Bischoff's  view,  but  until  the  adverse  results  obtained  by 
Clarke  and  others  from  the  study  of  the  human  corpus  luteum 
and  those  obtained  by  Jankowski  from  the  pig  have  been 
shown  to  be  incorrect,  the  question  as  to  the  invariable 
derivation  of  the  luteum  cells  from  the  stratum  granulosum 
must  be  left  open.     Since  it  is  held  that  both  the  granulosa 


THE    MATURATION    OF    THE    OVUM.        ^^  2/ 

and  theca  cells  are  derivatives  of  the  embryonic  ovarial 
epithelium  the  essential  differences  between  the  two  origins 
that  have  been  ascribed  to  the  liitenm  cells  may  not  be  so 
great  as  has  been  supposed. 

The  prevalent  tendency  toward  attributing  internal  secre- 
tions to  obscure  organs  has  not  allowed  the  corpus  luteum  to 
escape,  and  it  has  been  suggested  (Marshall  and  Jolly)  that  it 
provides  a  secretion  which  is  essential  for  the  changes  taking 
place  during  the  fixation  of  the  embryo  and  for  its  develop- 
ment during  the  early  stages.  In  support  of  this  view  it  has 
been  found  that  if  the  ovaries  were  removed  from  rats  and 
bitches  at  various  stages  after  impregnation,  pregnancy  did 
not  continue  if  the  operation  were  performed  during  its  earlier 
stages. 

The  Maturation  of  the  Ovum. — Returning  now  to  the 
ovum,  it  has  been  shown  that  at  the  time  of  its  extrusion 
from  the  Graafian  follicle  it  is  not  equivalent  to  a  sperma- 
tozoon but  to  a  primary  spermatocyte,  and  it  may  be  remem- 
bered that  such  a  spermatocyte  becomes  converted  into  a 
spermatozoon  only  after  it  has  undergone  two  divisions, 
during  which  there  is  a  reduction  of  the  number  of  the 
chromosomes  to  one  half  the  number  characteristic  for  the 
species. 

Similar  divisions  and  a  similar  reduction  of  the  chromo- 
somes occur  in  the  case  of  the  ovum,  constituting'  what  is 
termed  its  maturation.  The  phenomena  have  not  as  yet 
been  observed  in  human  ova,  and,  indeed,  among  mammals 
only  with  any  approach  to  completeness  in  the  mouse 
(Sobotta),  and- guinea-pig  (Rubaschkin)  ;  but  they  have 
been  observed  in  so  many  other  forms,  both  vertebrate  and 
inverte1:)rate,  and  present  in  all  cases  so  much  uniformity 
in  their  general  features,  that  there  can  be  little  question  as 
to  their  occurrence  in  the  human  ovum. 

In  typical  cases  the  ovum  (the  primary  oocyte)  under- 
goes a  division  in  the  prophases  of  which  the  chromatin 
aggregates  to  form  half  as  many  tetrads  as  there  are  chro- 


28 


THE    MATURATION    OF    THE    OVUM. 


mosomes  in  the  somatic  cells  (Fig.  13,  oc^)  and  at  the  meta- 
phase  a  dyad  from  each  tetrad  passes  into  each  of  the  two 
cells  that  are  formed.  These  two  cells  (secondary  oocytes) 
are  not,  however,  of  the  same  size;  one  of  them  is  almost 


Fig.    13. — Diagram    Illustrating    the    Reduction  '  of    the    Chromo- 
somes DURING  the  Maturation  of  the  Ovum. 
0,  Ovum ;  oc\  oocyte  of  the  first  generation ;  oc",  oocyte  of  the  second 
generation ;  p,  polar  globule. 

as  large  as  the  original  primary  oocyte  and  continues  to  be 
called  an  ovum  {oc^),  while  the  other  is  very  small  and  is 
termed  a  polar  globule  (p).  A  second  division  of  the  ovum 
(juickly  succeeds  the  first    (Fig.    13,   oc^),  and  each  dyad 


THE    MATURATION    OF    THE    OVUM.  29 

gives  a  single  chromosome  to  each  of  the  two  cells  which 
result,  so  that  each  of  these  cells  possesses  half  the  number 
of  chromosomes  characteristic  for  the  species.  The  second 
division,  like  the  first,  is  unequal,  one  of  the  cells  being 
relatively  very  large  and  constituting  the  mature  ovum, 
while  the  other  is  small  and  is  the  second  polar  globule. 
Frecjuently  the  first  polar  globule  divides  during  the  forma- 
tion of  the  second  one,  a  reduction  of  its  dyads  to  single 
chromosomes  taking  place,  so  that  as  the  final  result  of  the 
maturation  four  cells  are  formed  (Fig.  13),  the  mature 
ovum  (0),  and  three  polar  globules  (p),  each  of  which  con- 
tains half  the  number  of  chromosomes  characteristic  for 
the  species. 

The  similarity  of  the  maturation  phenomena  to  those 
of  spermatogenesis  may  be  perceived  from  the  following 
diagram : 

Sperniato- 
Oocyte  I  I       )  {      )  '^y'e  1 


Oocyte  n  C)  O  ()  CJ  ^%TeT 


Ovum  OO  00  00  00  Spermatids 

Polar  globules 

In  both  processes  the  number  of  cells  produced  is  the  same 
and  in  both  there  is  the  same  reduction  of  the  chromosomes. 
But  while  each  of  the  four  spermatids  is  functional,  the 
three  polar  globules  are  non- functional,  and  are  to  be  re- 
garded as  abortive  ova,  formed  during  the  process  of  reduc- 
tion of  the  chromosomes  only  to  undergo  degeneration.  In 
other  words,  three  out  of  every  four  potential  ova  sacrifice 


30 


THE    MATURATION    OF    THE    OVUM. 


themselves  in  order  that  the  fourth  may  have  the  bulk,  that 
is  to  say,  the  amount  of  nutritive  material  and  cytoplasm 
necessary  for  successful  development. 

In  the  mouse,  which  for  the  present  must  be  taken  as 
type  of  the  mammalia,  the  majority  of  ova  show  an  appar- 
ent departure  from  the  processes  just  described.  The  num- 
ber of  chromosomes  occurrino-  in  the  somatic  cells  of  the 


Fig.  14. — Ovum  of  a  Mouse  Showing  the  Maturation  Spindle. 

The  ovum  is  enclosed  by  the  zona  peHucida  (s.p),  to  which  the  cells  of 

the  corona  radiata  are  still  attached. —  (Sobotta.) 

mouse  is  twenty- four.  The  first  maturation  spindle  (Fig. 
14)  possesses  twelve  chromosomes,  which  from  analogy 
with  the  lower  forms  may  be  assumed  to  be  tetrads,  and 
during  the  metaphase  each  chromosome  divides  transversely, 
the  polar  globule  receiving  twelve  chromosomes,  presum- 
ably dyads,  while  twelve  remain  within  the  ovum.     So  far 


THE    FERTILIZATION    OF    THE    OVUM.  3 1 

the  process  is  essentially  typical,  but  in  at  least  75  per  cent, 
of  the  ova  examined  only  one  polar  globule  could  be  ob- 
served. In  the  remaining  25  per  cent,  two  polar  globules 
occurred,  the  twelve  chromosomes  again  dividing  trans- 
versely, so  that  the  second  polar  globule  and  the  ovum  each 
received  twelve  chromosomes  and  the  reduction  was  typical. 
Recent  observations,  however,  favor  the  idea  that  no  real 
deviation  from  the  normal  reduction  phenomena  occurs  in 
the  ovum  of  the  mouse.  Gerlach  holds  that  the  failure  of 
the  second  polar  globule  does  not  indicate  the  non-occur- 
rence of  the  second  maturation  mitosis,  but  merely  that 
this,  owing  to  the  late  entrance  of  a  spermatozoon  develops 
in  such  a  plane  or  in  such  a  position  that  the  pole  of  the 
spindle  does  not  reach  the  surface  of  the  ovum,  the  polar 
globule,  consequently,  not  being  extruded,  but  remaining 
within  the  ovum,  where  its  chromosomes  later  degenerate. 
Kirkham,  however,  goes  even  farther,  and  maintains  that 
the  second  polar  globule  is  formed  in  all  fertilized  ova,  the 
first  one  in  the  meantime,  however,  having  escaped  through 
the  zona  pellucida  in  the  majority  of  the  ova,  so  that  the 
appearance  of  but  a  single  globule  is  presented.  If  this 
last  view  be  correct  then  the  maturation  phenomena  of  the 
mouse's  ovum  are  perfectly  typical. 

The  Fertilization  of  the  Ovum. — It  is  perfectly  clear 
that  the  reduction  of  the  chromosomes  in  the  germ  cells 
cannot  very  long  be  repeated  in  successive  generations  unless 
a  restoration  of  the  original  number  takes  place  occasion- 
ally, and,  as  a  matter  of  fact,  such  a  restoration  occurs  at 
the  very  beginning  of  the  development  of  each  individual, 
being  brought  about  by  the  union  of  a  spermatozoon  with 
an  ovum.  This  union  constitutes  what  is  known  as  the 
fertilization  of  the  ovum. 

The  fertilization  of  the  human  ovum  has  not  yet  been 
observed,  but  the  phenomenon  has  been  repeatedly  studied 


32  THE    FERTILIZATION    OF    THE    OVUM. 

in  lower  forms,  and  a  thorough  study  of  the  process  has 
been  made  on  the  mouse  by  Sobotta,  whose  observations  are 
taken  as  a  basis  for  the  following  account. 

The  maturation  of  the  ovum  is  quite  independent  of  fer- 
tilization, but  in  many  forms  the  penetration  of  the  sperma- 
tozoon into  the  ovum  takes  place  before  the  maturation 
phenomena  are  completed.  This  is  the  case  with  the  mouse. 
A  spermatozoon  makes  its  way  through  the  zona  pellucida 
and  becomes  embedded  in  the  cytoplasm  of  the  ovum  and 
its  tail  is  quickly  absorbed  by  the  cytoplasm  while  its  nucleus 
and  probably  the  middle-piece  persist  as  distinct  structures. 
As  soon  as  the  maturation  divisions  are  completed  the 
nucleus  of  the  ovum,  now  termed  the  female  pronucleus 
(Fig.  15,  ek),  migrates  toward  the  center  of  the  ovum, 
and  is  now  destitute  of  an  archoplasm  sphere  and  centro- 
some,  these  structures  having  disappeared  after  the  com- 
pletion of  the  maturation  divisions.  The  spermatozoon 
nucleus,  which,  after  it  has  penetrated  the  ovum,  is  termed 
the  male  pronucleus  (spk),  may  lie  at  first  at  almost  any 
point  in  the  peripheral  part  of  the  cytoplasm,  and  it  now 
begins  to  approach  the  female  pronucleus,  preceded  by  the 
middle-piece,  which  becomes  an  archoplasm  sphere  with  its 
contained  centrosome  and  is  surrounded  by  astral  rays. 
The  two  pronuclei  finally  come  into  contact  near  the  center 
of  the  ovum,  forming  what  is  termed  the  segmentation 
nucleus  (Fig.  15),  and  the  archoplasm  sphere  and  centro- 
some which  have  been  introduced  with  the  spermatozoon 
undergo  division  and  the  two  archoplasm  spheres  so  formed 
migrate  to  opposite  poles  of  the  segmentation  nucleus,  an 
amphiaster  forms  and  the  compound  nucleus  passes  through 
the  various  prophases  of  mitosis.  Since,  in  the  mouse,  the 
male  and  female  i3ronuclei  have  each  contributed  twelve 
chromosomes,  the  equatorial  plate  of  the  mitosis  is  com- 
posed of  twenty-four  chromosomes,  the  number  character- 
istic for  the  species  being  thus  restored. 


THE    FERTILIZATION    OF    THE    OVUM. 


33 


■M 


rko 


©: 


-r\ 


e\L- 


•9P 

4      »• 


.     # 


o 


'  .^ 


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#-    <!  '^;. 


.    *■    -^  «•    .  * 


.   «" 


t^*        V  -f,     \_  ^■ 


Fig.    15. — Six   Stages  in  the  Process  of  Fertilization   of  the 
Ovum  of  a  Mouse. 
After  the  first  stage  figured  it  is  impossible  to  determine  which  of  the 
two  nuclei  represents  the  male  or  female  pronucleus,     ek,  Female 
pronucleus ;   rki  and   rk2,  polar  globules ;    spk,  male   pronucleus. — 
{Sobotta.) 


34  THE    FERTILIZATION    OF    THE    OVUM. 

It  seems  to  be  a  rule  that  but  one  spermatozoon  pene- 
trates the  ovnm.  Many,  of  course,  come  into  contact  with 
it  and  endeavor  to  penetrate  it,  but  so  soon  as  one  has  been 
successful  in  its  endeavor  no  further  penetration  of  others 
occurs.  The  reasons  for  this  are  in  most  cases  obscure; 
experiments  on  the  ova  of  invertebrates  have  shown  that 
the  subjection  of  the  ova  to  abnormal  conditions  which 
impair  their  vitality  favors  the  penetration  of  more  than  a 
single  spermatozoon  {polyspermy) ,  and,  indeed,  it  appears 
that  in  some  forms,  such  as  the  common  newt  (Diemycty- 
lus),  polyspermy  is  the  rule,  only  one  of  the  spermatozoa, 
however,  which  have  penetrated  uniting  with  the  female 
pronucleus,  the  rest  being  absorbed  by  the  cytoplasm  of 
the  ovum. 

Fertilization  marks  the  beginning  of  development,  and 
it  is  therefore  important  that  something  should  be  known 
as  to  where  and  when  it  occurs.  It  seems  probable  that 
in  the  human  species  the  spermatozoa  usually  come  into 
contact  with  the  ovum  and  fertilization  ensues  in  the  upper 
part  of  the  Fallopian  tubes,  and  the  occurrence  of  extra- 
uterine pregnancy  (see  p.  21)  seems  to  indicate  that  occa- 
sionally the  ovum  may  be  fertilized  even  before  it  has  been 
received  into  the  tube. 

It  is  evident,  then,  that  when  fertilization  is  accomplished 
the  spermatozoon  must  have  traveled  a  distance  of  about 
twenty- four  centimeters,  the  length  of  the  upper  part  of 
the  vagina  being  taken  to  be  about  5  cm.,  that  of  the  uterus 
as  7  cm.,  and  that  of  the  tube  as  12  cm.  A  considerable 
interval  of  time  is  required  for  the  completion  of  this  jour- 
ney, even  though  the  movement  of  the  spermatozoon  be 
tolerably  rapid.  The  observations  of  Henle  and  Hensen 
indicate  that  a  spermatozoon  may  progress  in  a  straight 
line  at  about  the  rate  of  from  1.2  to  2.7  mm.  per  minute, 
while  Lott  finds  the  rate  to  be  as  high  as  3.6  mm.     Assum- 


THE    FERTILIZATION    OF    THE    OVUM.  35 

ing  the  rate  of  progress  to  be  about  2.5  mm.  per  minute, 
the  time  required  by  the  spermatozoon  to  travel  from  the 
upper  part  of  the  vagina  to  the  upper  part  of  a  Fallopian 
tube  will  be  about  one  and  a  half  hours  (Strassman).  This, 
however,  assumes  that  there  are  no  obstacles  in  the  way  of 
the  rapid  progress  of  the  spermatozoon,  which  is  not  the 
case,  since,  in  the  first  place,  the  irregularities  and  folds  of 
the  lining  membrane  of  the  tube  render  the  path  of  the 
spermatozoon  a  labyrinthine  one,  and,  secondly,  the  action 
of  the  cilia  of  the  epithelium  of  the  tube  and  uterus  being 
from  the  ostium  of  the  tube  toward  the  os  uteri,  it  will 
greatly  retard  the  progress;  furthermore,  it  is  presumable 
that  the  rapidity  of  movement  of  the  spermatozoon  dimin- 
ishes after  a  certain  interval  of  time.  It  seems  probable, 
therefore,  that  fertilization  does  not  occur  for  some  hours 
after  coition,  even  providing  an  ovum  is  in  the  tube  await- 
ing the  approach  of  the  spermatozoon. 

But  this  condition  is  not  necessarily  present,  and  con- 
sequently the  question  of  the  duration  of  the  vitality  of 
the  sperm  cell  becomes  of  importance.  Ahlfeld  has  found 
that,  when  kept  at  a  proper  temperature,  a  spermatozoon 
will  retain  its  vitality  outside  the  body  for  eight  days,  and 
Diihrssen  reports  a  case  in  which  living  spermatozoa  were 
found  in  a  Fallopian  tube  removed  from  a  patient  who  had 
last  been  in  coitu  about  three  and  a  half  weeks  previously. 
As  regards  the  duration  of  the  vitality  of  the  ovum  less 
accurate  data  are  available.  Hyrtl  found  an  apparently 
normal  ovum  in  the  uterine  portion  of  the  left  tube  of  a 
female  who  died  three  days  after  the  occurrence  of  her 
second  menstruation,  and  Issmer  estimates  the  duration  of 
the  capacity  for  fertilization  of  an  ovum  to  be  about  sixteen 
days. 

It  is  evident,  then,  that  even  when  the  exact  date  of  the 
coitus  which  led  to  the  fertilization  is  known,  the  actual 


36  SUPERFETATION. 

moment  of  the  latter  process  can  only  be  approximated, 
and  in  the  immense  majority  of  cases  it  is  necessary  to 
rely  upon  the  date  of  the  last  menstruation  for  an  estima- 
tion of  the  probable  date  of  parturition.  And  by  this 
method  the  possibilities  for  error  are  much  greater.  It 
has  been  seen  that  ovulation  usually,  though  not  invariably, 
is  associated  with  menstruation,  but  it  is  uncertain  whether 
the  ovum  whose  fertilization  has  resulted  in  a  pregnancy 
was  expelled  from  its  follicle  during  the  last  menstrual 
period  which  occurred,  or  during  or  just  preceding  the  first 
omitted  period.  Both  views  have  been  advocated,  but  it 
seems  probable  that  the  latter  case  is  the  more  frecjuent, 
the  fertilized  ovum  being  one  which  has  been  expelled  from 
its  follicle  subsecjuent  to  the  last  menstruation  which  oc- 
curred. The  duration  of  pregnancy  is  normally  ten  lunar 
or  about  nine  calendar  months  and  it  is  customary  to  esti- 
mate the  probable  date  of  parturition  as  nine  months  and 
seven  days  from  the  last  menstruation.  From  what  has 
been  said,  it  is  clear  that  any  such  estimation  can  be  de- 
pended upon  only  as  an  approximation,  the  possible  varia- 
tion from  it  being  considerable. 

Superfetation. — The  occasional  occurrence  of  twin  fetuses 
in  different  stages  of  development  has  suggested  the  possi- 
bility of  the  fertilization  of  a  second  ovum  as  the  result  of  a 
coition  at  an  appreciable  interval  of  time  after  the  first  ovum 
has  started  upon  its  development.  There  seems  to  be  good 
reason  for  believing  that  many  of  the  cases  of  supposed  super- 
fetation^  as  this  phenomenon  is  termed,  are  instances  of  the 
simultaneous  fertilization  of  two  ova,  one  of  which,  for  some 
cause  concerned  with  the  supply  of  nutrition,  has  later  failed 
to  develop  as  rapidly  as  the  other.  At  the  same  time,  how- 
ever, even  although  the  phenomenon  may  be  of  rare  occurrence, 
it  is  by  no  means  impossible,  for  occasionally  a  second  Graafian 
follicle,  either  in  the  same  or  the  other  ovary,  may  be  so  near 
maturity  that  its  ovum  is  extruded  soon  after  the  first  one, 
and  if  the  development  of  the  latter  and  the  incidental  changes 
in  the  uterine  mucous  membrane  have  not  proceeded  so  far 
as  to  prevent  the  access  of  the  spermatozoon  to  the  ovum. 


LITERATURE.  37 

its  fertilization  and  development  may  ensue.  The  changes, 
however,  which  prevent  the  passage  of  the  spermatozoon  are 
completed  early  in  development  and  the  difference  between 
the  normally  developed  embryo  and  that  due  to  superfetation 
will  be  comparatively  small,  and  will  become  less  and  less 
evident  as  development  proceeds,  provided  that  the  supply  of 
nutrition  to  both  embryos  is  equal. 

LITERATURE. 

E.  Ballowitz  :  "  Untersuchungeii  iiber  die  Struktur  der  Spermatozoen," 

No.  4.     Zcitschr.  fur  luisscnsch.  Zool.,  \a\,  1891. 
K.   VON    Bardeleben  :    "  Beitrage   zur   Histologic   des   Hodens   und   zur 

Spermatogenese  beim  Menschen,"  Arcliiv  fiir  Anat.   und  Physiol., 

Anat.  Abtli^,  Supplement,  1897. 
Th.  Boveri:  "  Befruchtung,"  Ergebnisse  der  Anat.  und  Entzvicklungs- 

gescJi.,  I,  1892. 
J.  G.   Clark  :   "  Ursprung,  Wachsthum  und  Ende   des  Corpus  luteum 

nacli   Beobachtungen   am   Ovarium  des   Schweines  und   des   Men- 
schen," Archiv  fiir  Anat.  und  Physiol.,  Anat.  Abth.,  1898. 
L.  Gerlach  :  "  Ucber  die  Bildung  der  Richtungskorper  bci  Mus  muscu- 

lus,"  Wiesbaden,   1906. 
V/.  Heape:  "The  Sexual  Season  of  Mammals  and  the  Relation  of  the 

Prooestrum  to   Menstruation,"    Quart.   Journ.  Micros.   Sci.,  N.   S., 

XLiv,  1901    (contains  very  full  bibliography). 
O.  Hertwig  :   "  Vergleich  der  Ei-  und  Samenbildung  bei  Nematoden," 

Archiv  fiir  mikrosk.  Anat.,  xxxvr,  1890. 
J.  Jankowski  :  "  Beitrag  zur  Entstehung  des  Corpus  luteum  der  Sau- 

getiere,"  Arch.  f.  mikr.  Anat.,  lxiv^  1904. 
W.  B.  Kirkham  :  "The  Maturation  of  the  Mouse  Egg,"  Biol.  Bulletin, 

XII,  1907. 
M.  voN  Lenhossek  :  "  Untersuchungen  iiber  Spermatogenese,"  Archiv 

fiir  mikrosk.  Anat.,  ia,  1898. 

F.  H.  A.  Marshall:  "The  CEstrus  Cycle  and  the  Formation  of  the 

Corpus  luteum  in  the  Sheep,"  Philos.  Trans.,  Ser.  B,  cxcvi,  1904. 
F.   H.   A.   Marshall:    "The   Development   of  the   Corpus   luteum:    a 
Review,"  Otiart.  Journ.  Micros.  Sci.,  N.  S.,  xlix,  1906. 

F.  Meves  :    "  Ueber    Struktur   und    Histogenese    der    Samenfaden    des 

Meerschweinchens,"  Archiv  fiir  mikrosk.  Anat.,  liv,  1899. 
W.   Nagel:   "Das  menschliche   Ei,"  Archiv  fiir  mikrosk.  Anat.,  xxxi. 
1888. 

G.  Niessing:    "Die    Betheilignng   der   Centralkorper   und    Sphare   am 

Aufbau   des   Samenfadens  bei   Saugethieren,"  Archiv  fiir  mikrosk. 
Anat.,  xLviii,  1896. 


38  LITERATURE. 

W.    RuBASCHKiN :    "  Ueber    die   Reifungs-   iind    Bef  ruchtungs-processe 

des  Meerschweincheneies,"  Anat.  Hcfte,  xxix,  1905. 
J.    SoBOTTA :    "  Die   Befruchtung   und    Furchung   des   Eies    der   Mans," 

Archiv  fi'ir  niikrosk.   Anat.,  xlv,   1895. 
J.   SoBOTTA :   "  Ueber   die   Bildung   des   Corpus   luteum  bei   der   Mans," 

Archiv  fiir  niikrosk.  Anat.,  xlvii,  1897. 
J.    SoBOTTA :    "  Ueber   die    Bildung   des    Corpus   luteum   beim   Meerscb- 

weinchen,"  Anat.  Hefte,  xxxii,  igo6. 
P.  Strassmann  :  "  Beitrage  zur  Lehre  von  der  Ovulation,  Menstruation 

und  Conception,"  Archiv  fur  GynackoL,  lii,  1896. 
W.  Waldeyer:  "  Eierstock  und  Ei,"  Leipzig,  1870. 


CHAPTER  II. 

THE  SEGMENTATION  OF  THE  OVUM  AND  THE 
FORMATION  OF  THE  GERM  LAYERS. 

Segmentation. — The  union  of  the  male  and  female  pro- 
nuclei has  already  been  described  as  being  accompanied  by 
the  formation  of  a  mitotic  spindle  which  produces  a  division 
of  the  OATini  into  two  cells.  Tliis  first  division  is  succeeded 
at  more  or  less  regular  inter\-als  by  others  until  a  mass  of 
cells  is  produced  in  which  a  differentiation  eventually  ap- 
pears. These  divisions  of  the  ovum  constitute  what  is 
termed  its  segmentation. 

The  mammalian  o\Tim  has  behind  it  a  long  line  of  evolu- 
tion, and  even  at  early  stages  in  its  development  it  exhibits 
peculiarities  which  can  only  be  reasonably  explained  as  an 
inheritance  of  past  conditions.  One  of  the  most  potent 
factors  in  modifying  the  character  of  the  segmentation  of 
the  OATun  is  the  amount  of  food  A-olk  which  it  contains,  and 
it  seems  to  be  certain  that  the  immediate  ancestors  of  the 
mammalia  were  forms  whose  ova  contained  a  considerable 
amomit  of  yolk,  many  of  the  peculiarities  resulting  from  its 
presence  being  still  clearly  indicated  in  the  early  develop- 
ment of  the  almost  5'^olkless  mammalian  o\Tim.  To  give 
some  idea  of  the  peculiarities  which  result  from  the  presence 
of  considerable  amounts  of  yolk  it  will  be  well  to  compare 
tlie  processes  of  segmentation  and  dift'erentiation  seen  in  ova 
with  different  amounts  of  it. 

A  little  below  the  scale  of  the  vertebrates  proper  is  a 
form,  Aniphioxus,  which  possesses  an  almost  yolkless  ovum, 
presenting  a  simple  process  of  development.     The  fertilized 

39 


40 


THE    SEGMENTATION    OF    THE    OVUM. 


ovnm  of  Ainphio.xus  in  its  first  division  separates  into  two 
similar  and  equal  cells,  and  these  are  made  four  (Fig.  i6, 
A)  by  a  second  plane  of  division  which  cuts  the  previous 
one  at  right  angles.  A  third  plane  at  right  angles  to  both 
the  preceding  ones  brings  about  an  eight-celled  stage  (Fig. 
i6,  B),  and  further  divisions  result  in  the  formation  of  a 
large  number  of  cells  which  arrange  themselves  in  the  form 
of  a  hollow  sphere  which  is  known  as  a  blast ula  (Fig.  i6,  E) . 


Fig.   i6. — ^Stages  in  the  Segmentation  of  Ainphioxus. 

A,  Four-celled  stage;  B,  eight-celled  stage;  C,  sixteen-celled  stage;  D, 

early  blastula;  E,  blastula;  F,  section  of  blastula. —  (Hatschck.) 


The  minute  amount  of  yolk  which  is  present  in  the  ovum 
of  Ainpliioxus  collects  at  an  early  stage  of  the  segmentation 
at  one  pole  of  the  ovum,  the  cells  containing  it  being  some- 
what larger  than  those  of  the  other  pole  (Fig.  i6,  B),  and 
in  the  blastula  the  cells  of  one  pole  are  larger  and  more 
richly  laden  with  yolk  than  those  of  the  other  pole  (Fig. 
i6,  F).  If,  now,  the  segmenting  ovum  of  an  Amphibian 
be  examined,  it  will  be  found  that  a  very  much  greater 
amount  of  yolk  is  present  and,  as  mAmphioxus,  it  is  located 
especially  at  one  pole  of  the  ovum.  The  first  three  planes 
of   segmentation   have   the   same   relative  positions   as   in 


THE    SEGMENTATION    OF    THE    OVUM.  4^ 

Amphioxiis  (Fig.  i6),  but  one  of  the  tiers  of  cells  of  the 
eight-celled  stage  is  very  much  smaller  than  the  other  (Fig., 
17,  B).  In  the  subsequent  stages  of  segmentation  the  small 
cells  of  the  upper  pole  divide  more  rapidly  than  the  larger 
ones  of  the  lower  pole,  the  activity  of  the  latter  seeming  to 
be  retarded  by  the  accumulation  of  the  yolk,  and  the  result- 


ii.tj^'38«p*,^^ 


B 


C  D 

Fig.    17. — Stages    in    the    Segmentation    of    Ainblystorna. —  (Eyclcs- 

hymer.) 

ing  blastula  (Fig.  17,  D)  shows  a  very  decided  difference 
in  the  size  of  the  cells  of  the  two  poles. 

In  the  ova  of  reptiles  and  birds  the  amount  of  yolk  stored 
up  in  the  ovum  is  very  much  greater  even  than  in  the  am- 
phibia, and  it  is  aggregated  at  one  pole  of  the  ovum  of 
which  it  forms  the  principal  mass,  the  yolkless  protoplasm 
appearing  as  a  small  disk  upon  the  surface  of  a  relatively 
5 


42 


THE    SEGMENTATION    OF   THE    OVUM. 


huge  mass  of  yolk.  The  inertia  of  this  mass  of  nutritive 
material  is  so  great  that  the  segmentation  is  confined  to  the 
small  yolkless  disk  of  protoplasm  and  affects  consequently 
only  a  portion  of  the  entire  ovum.  To  distinguish  this  form 
of  segmentation  from  that  which  affects  the  entire  ovum  it 


Fig.    i8. — Four    Stages    in    the    Segmentation    of   the    Blastoderm 
OF  the  Chick. —  (Costc.) 

is  termed  mcrohlastic  segmentation,  the  other  form  being 
known  as  Iwloblastic. 

In  the  ovum  of  a  turtle  or  a  bird  the  first  plane  of  seg- 
mentation crosses  the  protoplasmic  disk,  dividing  it  into 
two  practically  equal  halves,  and  the  second  plane  forms 
at  approximately  right  angles  to  the  first  one,  dividing  the 
disk  into  four  quadrants  (Fig.  i8,  A).  The  third  division, 
like  the  two  which  precede  it,  is  radial  in  position,  while 


THE    SEGMENTATION    OF    THE    OVUM.  43 

the  fourth  is  circular  and  cuts  off  the  inner  ends  of  the  six 
cehs  previously  formed  (Fig.  i8,  D).  The  disk  now  con- 
sists of  six  central  smaller  cells  surrounded  by  six  larger 
peripheral  ones.  Beyond  this  period  no  regularity  can  be 
discerned  in  the  appearance  of  the  segmentation  planes;  but 
radial  and  circular  divisions  continuing  to  form,  the  disk 
becomes  divided  into  a  large  number  of  cells,  those  at  the 
center  being  much  smaller  than  those  at  the  periphery.  In 
the  meantime,  however,  the  smaller  central  cells  have  begun 


Fig.    19. — Diagram     Illustrating    a    Section    of    the    Ovum    of    a 
Reptile    at    a    Stage    Corresponding    to    the    Blastula    of    an 


Amphibian. 


bl.  Blastoderm;    Y,  yolk-mass. 


to  divide  in  planes  parallel  to  the  surface  of  the  disk,  which, 
from  being  a  simple  plate  of  cells,  thus  becomes  a  discoidal 
cell-mass. 

During  the  segmentation  of  the  disk  it  has  increased 
materially  in  size,  extending  further  and  further  over  the 
surface  of  the  yolk,  into  the  substance  of  which  some  of 
the  lower  cells  of  the  discoidal  cell-mass  have  penetrated. 
A  comparison  of  the  diagram  (Fig.  19)  of  the  ovum  of 
a  reptile  at  about  this  stage  of  development  with  the  figure 
of  the  amphibian  blastula   (Fig.    17,  D)   will  indicate  the 


44  THE    SEGMENTATION    OF    THE    OVUM. 

similarity  between  the  two,  the  large  yolk-mass  of  the  rep- 
tile (F)  with  the  scattered  cells  which  it  contains  corre- 
sponding to  the  lower  pole  cells  of  the  amphibian  blastula, 
the  central  cavity  of  which  is  practically  suppressed  in  the 
reptile.  Beyond  this  stage,  however,  the  similarity  becomes 
more  obscured.  The  peripheral  cells  of  the  disk  continue 
to  extend  over  the  surface  of  the  yolk  and  finally  completely 
enclose  it,  forming  an  enveloping  layer  which  is  completed 
at  the  upper  pole  of  the  tgg  by  the  discoidal  cell-mass,  or, 
as  it  is  usually  termed,  the  blastoderm. 

Turning  now  to  the  mammalia,*  it  will  be  found  that 
the  ovum  in  the  great  majority  is  almost  or  cjuite  as  desti- 
tute of  food  yolk  as  is  the  ovum  of  Amphioxus,  with  the 
result  that  the  segmentation  is  of  the  total  or  holoblastic 
type.  It  does  not,  however,  proceed  with  that  regularity 
which  marks  the  segmentation  of  Amphioxus  or  an  am- 
phibian, but  while  at  first  it  divides  into  two  slightly  unequal 
cells  (Fig'.  20),  thereafter  the  divisions  become  irregular, 
three-celled,  four-celled,  five-celled,  and  six-celled  stages 
having  been  observed  in  various  instances.  Nor  is  the  re- 
sult of  the  final  segmentation  a  hollow  vesicle  or  blastula, 
but  a  solid  mass  of  cells,  termed  a  morula,  is  formed.  This 
structure  is  not,  however,  comparable  to  the  blastula  of  the 
lower  forms,  but  corresponds  to  a  stage  of  reptilian  devel- 
opment a  little  later  than  that  shown  in  Fig.  19,  since,  as 
will  be  shown  directly,  the  cells  corresponding  to  the  blasto- 
derm and  the  enveloping  layer  are  already  present.  There 
is,  then,  no  blastula  stage  in  the  mammalian  development. 

Difi^erentiation  now  begins  by  the  peripheral  cells  of  the 
morula  becoming  less  spherical  in  shape  and  later  forming 
a  layer  of  flattened  cells,  the  enveloping  layer,  surrounding 


*  The  segmentation  of  the  human  ovum  has  not  yet  been  observed ; 
what  follows  is  based  on  what  occurs  in  the  ovum  of  the  rabbit,  mole, 
and  especially  of  a  bat  (Van  Beneden). 


THE    SEGMENTATION    OF    THE    OVUM. 


45 


the  more  spherical  central  cells  (Fig.  21,  A).  In  the  latter 
vacuoles  now  make  their  appearance,  especially  in  those  cells 
which  are  nearest  what  may  be  regarded  as  the  lower  pole 
of  the  ovum  (Fig.  21,  C),  and  these  vacuoles,  gradually 
increasing  in  size,  eventually  become  confluent,  the  condi- 


FiG.    20. — Four    Stages    in    the    Segmentation    of   the    Ovum    of   a 

Mouse. 
X,   Polar  globule. —  (Sobotta.) 

tion  represented  in  Fig.  21,  D,  being  produced.  At  this 
stage  the  ovum  consists  of  an  enveloping  layer,  enclosing  a 
cavity  which  is  equivalent  to  the  yolk-mass  of  the  reptilian 
ovum,  the  vacuolization  of  the  inner  cells  of  the  morula 
representing  a  belated  formation  of  yolk.  On  the  inner 
surface  of  the  enveloping  layer,  at  what  may  be  termed  the 


46 


THE    SEGMENTATION    OF    THE    OVUM. 


^jm^*'S^:^i 


^ 


"v^^S^*-*- 


/S: 


•/?X"/ 


Fir.    21  —Later   Stages   in   the   Segmentation   of  the   Ovum   of  a 

Bat. 
A,  C,  and  D  are  sections,  B  a  surface  view.— (Fan  Bcncdcn.) 


THE    SEGMENTATION    OF    THE    OVUM.  4/ 

upper  pole  of  the  ovum,  is  a  mass  of  cells  projecting  into 
the  yolk-cavity  and  forming  what  is  known  as  the  inner 
cell-mass,  a  structure  comparable  to  the  blastoderm  of  the 
reptile.  In  one  respect,  however,  a  difference  obtains,  the 
inner  cell-mass  being  completely  enclosed  within  the  envel- 
oping cells,  which  is  not  the  case  with  the  blastoderm  of  the 
reptile.  That  portion  of  the  enveloping  layer  which  covers 
the  cell-mass  has  been  termed  Rauher's  covering  layer,  and 
probably  owes  its  existence  to  the  precocity  of  the  formation 
of  the  enveloping  layer. 

It  is  clear,  then,  that  an  explanation  of  the  early  stages 
of  development  of  the  mammalian  ovum  is  to  be  obtained 
by  a  comparison,  not  with  a  yolkless  ovum  such  as  that  of 
Ainphioxns,  but  with  an  ovum  richly  laden  with  yolk,  such 
as  the  meroblastic  ovum  of  a  reptile  or  bird.  In  these  forms 
the  nutrition  necessary  for  the  growth  of  the  embryo  and 
for  the  complicated  processes  of  development  is  provided 
for  by  the  storing  up  of  a  quantity  of  yolk  in  the  ovum, 
the  embryo  being  thus  independent  of  external  sources  for 
food.  The  same  is  true  also  of  the  lowest  mammalia,  the 
Monotremes,  which  are  egg-laying  forms,  producing  ova 
resembling  greatly  those  of  a  reptile.  When,  however,  in 
the  higher  mammals  the  nutrition  of  the  embryo  became 
provided  for  by  the  attachment  of  the  embryo  to  the  walls 
of  the  uterus  of  the  parent  so  that  it  could  be  nourished 
directly  by  the  parent,  the  storing  up  of  yolk  in  the  ovum 
was  unnecessary  and  it  became  a  holoblastic  ovum,  although 
many  peculiarities  dependent  on  the  original  meroblastic 
condition  persisted  in  its  development. 

Twin  Development. — As  a  rule,  in  the  human  species  but  one 
embryo  develops  at  a  time,  but  the  occurrence  of  twins  is  by 
no  means  infrequent,  and  triplets  and  even  quadruplets  occa- 
sionally are  developed.  The  occurrence  of  twins  may  be  due 
to  two  causes,  either  to  the  simultaneous  ripening  and  fertiliza- 
tion of  two  ova,  either  from  one  or  from  both  ovaries,  or  to  the 


48  TWIN    DEVELOPMENT   AND   DOUBLE    MONSTERS. 

separation  of  a  single  fertilized  ovum  into  two  independent 
parts  during  the  early  stages  of  development.  That  twins 
may  be  produced  by  this  latter  process  has  been  abundantly 
shown  by  experimentation  upon  developing  ova  of  lower  forms, 
each  of  the  two  cells  of  an  Amphioxus  ovum  in  that  stage  of 
development,  if  mechanically  separated,  completing  its  devel- 
opment and  producing  an  embryo  of  about  half  the  normal 
size. 

Double  Monsters  and  the  Duplication  of  Parts. — The  occa- 
sional occurrence  of  double  monsters  is  explained  by  an  imper- 
fect separation  into  two  parts  of  an  originally  single  embryo, 
the  extent  of  the  separation,  and  probably  also  the  stage  of 
development  at  w^hich  it  occurs,  determining  the  amount  of 
fusion  of  the  two  individuals  constituting  the  monster.  All 
gradations  of  separation  occur,  from  almost  complete  separa- 
tion, as  seen  in  such  cases  as  the  Siamese  twins,  to  forms  in 
which  the  two  individuals  are  united  throughout  the  entire 
length  of  their  bodies.  The  separation  may  also  affect  only 
a  portion  of  the  embryo,  producing,  for  instance,  double- 
faced  or  double-headed  monsters  or  various  forms  of  so-called 
parasitic  monsters ;  and,  finally,  it  may  affect  only  a  group  of 
cells  destined  to  form  a  special  organ,  producing  an  excess  of 
parts,  such  as  supernumerary  digits  or  accessory  spleens. 

It  has  been  observed  in  the  case  of  double  monsters  that  one 
of  the  two  fused  individuals  always  has  the  position  of  its 
various  organs  reversed,  it  being,  as  it  were,  the  looking- 
glass  image  of  its  fellow.  Cases  of  a  similar  situs  inversus 
viscerum,  as  it  is  called,  have  not  infrecjuently  been  observed 
in  single  individuals,  and  a  plausible  explanation  of  such  cases 
regards  them  as  one  of  a  pair  of  twins  formed  by  the  division 
of  a  single  embryo,  the  other  individual  having  ceased  to 
develop  and  either  having  undergone  degeneration  or,  if  the 
separation  was  an  incomplete  one,  being  included  within  the 
body  of  the  apparently  single  individual.  Another  explanation 
of  situs  inversus  has  been  advanced  (Conklin)  on  the  basis  of 
what  has  been  observed  in  certain  invertebrates.  In  some 
species  of  snails  situs  inversus  is  a  normal  condition  and  it 
has  been  found  that  the  inversion  may  be  traced  back  in  the 
development  even  to  the  earliest  segmentation  stages.  The 
conclusion  is  thereby  indicated  that  its  primary  cause  may 
reside  in  an  inversion  of  the  polarity  of  the  ovum,  evidence 
being  forthcoming  in  favor  of  the  view  that  even  in  the  ovum 
of  these  and  other  forms  there  is  probably  a  distinct  polar 
differentiation.     How  far  this  view  may  be  applicable  to  the 


FORMATION    OF    THE    GERM    LAYERS.  49 

mammalian  ovum  is  uncertain,  but  if  it  be  applicable  it  ex- 
plains the  phenomenon  of  inversion  without  complicating  it 
with  the  question  of  twin-formation. 

The  Formation  of  the  Germ  Layers. — During  the 
stages  which  have  been  described  as  belonging  to  the  seg- 
mentation period  of  development  there  has  been  but  little 
differentiation  of  the  cells.  In  Amphioxus  and  the  am- 
phibians the  cells  at  one  pole  of  the  blastula  are  larger  and 
more  yolk-laden  than  those  at  the  other  pole,  and  in  the 
mammals  an  inner  cell-mass  can  be  distinguished  from  the 
enveloping  cells,  this  latter  differentiation  having  been 
anticipated  in  the  reptiles  and  being  a  differentiation  of  a 
portion  of  the  ovum  from  which  alone  the  embryo  will 
develop  from  a  portion  which  will  give  rise  to  accessory 
structures.  In  later  stages  a  differentiation  of  the  inner 
cell-mass  occurs,  resulting  first  of  all  in  the  formation  of  a 
two-layered  or  diploblastic  and  later  of  a  three-layered  or 
triploblasfic  stage. 

Just  as  the  segmentation  has  been  shown  to  be  profoundly 
modified  by  the  amount  of  yolk  present  in  the  ovi!im  and  by 
its  secondary  reduction,  so,  too,  the  formation  of  the  three 
primitive  layers  is  much  modified  by  the  same  cause,  and 
to  get  a  clear  understanding  of  the  formation  of  the  triplo- 
blasfic condition  of  the  mammal  it  will  be  necessary  to  de- 
scribe briefly  its  development  in  lower  forms. 

In  AmpJiioxus  the  diploblastic  condition  results  from  the 
flattening  of  the  large-celled  pole  of  the  blastula  (Fig.  22, 
A),  and  finally  from  the  invagination  of  this  portion  of  the 
vesicle  within  the  other  portion  (Fig.  22,  B).  The  original 
single-walled  blastula  in  this  way  becomes  converted  into  a 
double-walled  sac  termed  a  gasfntia,  the  outer  layer  of 
w^hich  is  known  as  the  ectoderm  or  epiblasf  and  the  inner 
layer  as  the  eiidodenii  or  hypoblast.  The  cavity  bounded 
by  the  endoderm  is  the  primitive  gait  or  arclicnteron,  and 
6 


so 


FORMATION    OF    THE    GERM    LAYERS. 


the  Opening  by  which  this  communicates  with  the  exterior 
is  the  blastopore.  This  last  structure  is  at  first  a  very 
wide  opening-,  but  as  development  proceeds  it  becomes 
smaller,  and  finally  is  a  relatively  small  opening  situated  at 
the  posterior  extremity  of  what  will  be  the  dorsal  surface 
of  the  embryo. 

As  the  oval  embryo  continues  to  elongate  in  its  later 
development  the  third  layer  or  mesoderm  makes  its  appear- 
ance. It  arises  as  a  lateral  fold  {mp)  of  the  dorsal  surface 
of  the  endoderm  {en)  on  each  side  of  the  middle  line  as  indi- 


fi^^JZ  X 


"'^\^-, 


"^■^ 


A 


B 


Fig.  22. — Two  Stages  in  the  Gastrulation  of  Aniphioxus.- — (Morgan 

and  Hazen.) 


cated  in  the  transverse  section  shown  in  Fig.  23.  This  fold 
eventually  becomes  completely  constricted  ofif  from  the  en- 
doderm and  forms  a  hollow  plate  occupying  the  space 
between  the  ectoderm  and  endoderm,  the  cavity  which  it 
contains  being  the  body-cavity  or  ca^lom. 

In  the  amphibia,  where  the  amount  of  yolk  is  very  much 
greater  than  in  Amphioxus,  the  gastrulation  becomes  con- 
siderably modified.  On  the  line  where  the  large-  and  small- 
celled  portions  of  the  blastula  become  continuous  a  crescentic 
groove  appears  and,  deepening,  forms  an  invagination  (Fig. 


FORMATION    OF    THE    GERM    LAYERS. 


51 


24,  gc),  the  roof  of  which  is  composed  of  relatively  small 
yolk-containing  cells  while  its  floor  is  formed  by  the  large 
cells  of  the  lower  pole  of  the  blastula.  The  cavity  of  the 
blastula  is  not  sufficiently  large  to  allow  of  the  typical  in- 
vagination of  all  these 
large  cells,  so  that  they 
become  enclosed  by  the 
rapid  growth  of  the  ecto- 
derm cells  of  the  upper 
pole  of  the  ovum  over 
them.  Before  this  growth 
takes  place  the  blastopore 
corresponds  to  the  entire 
area  occupied  by  the  large 
yolk  cells,  but  later,  as  the 
grow^th  of  the  smaller  pic.  23.— Transverse  Section  of 
cells  gradually  encloses  Amphioxus  Embryo  with  Five 
,        ,  .       ,  Mesodermic  Pouches. 

the  larger  ones,  it  be- 
comes smaller  and  is 
finally  represented  by  a 
small  opening  situated  at 
what  will  be  the  hind  end  of  the  embryo. 

Soon  after  the  archenteron  has  been  formed  a  solid  plate 
of  cells,  eventually  splitting  into  two  layers,  arises  from  its 
roof  on  each  side  of  the  median  line  and  grows  out  into  the 
space  between  the  ectoderm  and  endoderm  (Fig.  25,  vik^ 
and  mk'^),  evidently  corresponding  to  the  hollow  plates 
formed  in  the  same  situations  in  Amphio.viis.  This  is  not, 
however,  the  only  source  of  the  mesoderm  in  the  amphibia, 
for  while  the  blastopore  is  still  quite  large  there  may  be 
found  surrounding  it  between  the  endoderm  and  ectoderm 
a  ring  of  mesodermal  tissue  (Fig.  24,  mes).  As  the  blas- 
topore diminishes  in  size  and  its  lips  come  together  and 
unite,  the  ring  of  mesoderm  forms  first  an  oval  and  then  a 
band  lying  beneath  the  line  of  closure  of  the  blastopore  and 
united  with  both  the  superjacent  ectoderm  and  the  subja- 


Ch,  Notochord;  d,  digestive  cavity; 
ec,  ectoderm ;  en,  endoderm ;  in, 
medullary  plate ;  fnp,  mesodermic 
pouch. —  (Hatschek.) 


52 


FORMATION    OF    THE    GERM    LAYERS. 


cent  endoderm.  This  line  of  fusion  of  the  three  germ  layers 
is  known  as  the  primitive  streak.  It  is  convenient  to  dis- 
tinguish the  mesoderm  of  the  primitive  streak  from  that 
formed  from  the  dorsal  wall  of  the  archenteron  by  speak- 
ing of  the  former  as  the  prostomial  and  the  latter  as  the 
gastral  mesoderm,  though  it  must  be  understood  that  the 


Fig.    24. — Section   through    a   Gastrula   of   Amhlystoma. 

dl,  Dorsal  lip  of  blastopore;  gc,  digestive  cavity;  gr,  area  of  mesoderm 

formation;   mes,  mesoderm. — {Eycleshymer.) 


two  are  continuous  immediately  in  front  of  the  definitive 
blastopore. 

In  the  reptilia  still  greater  modifications  are  found  in 
the  method  of  formation  of  the  germ  layers.  Before  the 
enveloping  cells  have  completely  surrounded  the  yolk-mass, 
a  crescentic  groove,  resembling  that  occurring  in  amphibia, 
appears  near  the  posterior  edge  of  the  blastoderm,  the  cells 
of  which,  in  front  of  the  groove,  arrange  themselves  in  a 
superficial  layer  one  cell  thick,  which  may  be  regarded  as 


FORMATION    OF    THE    GERM    LAYERS.  53 

the  ectoderm  (Fig.  26,  ec),  and  a  subjacent  mass  of  some- 
what scattered  cells.  Later  the  lowermost  cells  of  this  sub- 
jacent mass  arrange  themselves  in  a  continuous  layer,  con- 
stituting what  is  termed  the  primary  endoderm  (en^),  while 
the  remaining  cells,  aggregated  especially  in  the  region  of 
the  crescentic  groove,  form  the  prostomial  mesoderm  (prin). 
In  the  region  enclosed  by  the  groove  a  distinct  delimitation 
of  the  various  layers  does  not  occur,  and  this  region  forms 
the  primitive  streak.     The  groove  now  begins  to  deepen. 


Fig.  25. — Section  through  an  Embryo  Amphibian  (Triton)  of  22 
Days,  showing  the  Formation  of  the  Gastral  Mesoderm. 

ak,  Ectoderm ;  ch,  chorda  endoderm ;  dk,  digestive  cavity ;  ik,  endo- 
derm ;  mk^  and  nik',  splanchnic  and  somatic  layers  of  the  meso- 
derm.   D,  dorsal  and  V,  ventral. —  (Hcrtwig.) 

forming  an  invagination  of  secondary  endoderm,  the  extent 
of  this  invagination  being,  however,  very  different  in  dif- 
ferent species.  In  the  gecko  (Will)  it  pushes  forward  be- 
tween the  ectoderm  and  primary  endoderm  almost  to  the 
anterior  edge  of  the  blastoderm,  but  later  the  cells  forming 
its  floor,  together  with  those  of  the  primary  endoderm  im- 
mediately below,  undergo  a  degeneration,  the  roof  cells  at 
the  lateral  margins  of  the  invagination  becoming  contin- 


54  FORMATION    OF    THE    GERM    LAYERS. 

itous  with  the  persisting  portions  of  the  primary  endoderm 
(Fig.  27,  B).  This  layer,  foUowing  the  enveloping  cells 
in  their  growth  over  the  yolk-mass,  gradually  surrounds 
that  structure  so  that  it  comes  to  lie  within  the  archenteron. 
In  some  turtles,  on  the  other  hand,  the  disappearance  of  the 
floor  of  the  invagination  takes  place  at  a  very  early  stage 
of  the   infolding",   the  roof  cells   only  persisting  to  grow 


prm 


Tsefi 


B 


jprm 


Fig.   26. — Longitudinal   Sections   through   Embryos  of   the   Gecko, 

SHOWING     GaSTRULATION. 

ec,  Ectoderm ;   en,  secondary  endoderm ;   en  ,  primary  endoderm ;  prm, 
prostomial    mesoderm. —  (Will.) 

forward  to  form  the  dorsal  wall  of  the  archenteron.  This 
interesting  abbreviation  of  the  process  occurring  in  the 
gecko  indicates  the  mode  of  development  which  is  found  in 
the  mammalia. 

The  existence  of  a  prostomial  mesoderm  in  connection 
with  the  primitive  streak  has  already  been  noted,  and  when 
the  invagination  takes  place  it  is  carried  forward  as  a  nar- 


FORMATION    OF    THE    GERM    LAYERS. 


55 


row  band  of  cells  on  each  side  of  the  sac  of  secondary 
endoderm.  After  the  absorption  of  the  ventral  wall  of  the 
invagination  a  folding"  or  turning  in  of  the  margins  of  the 
secondary  endoderm  occurs  (Fig.  27),  whereby  its  lumen 
becomes  reduced  in  size  and  it  passes  off  on  each  side  into 
a  double  plate  of  cells  which  constitute  the  gastral  meso- 
derm.    Later  these  plates  separate   from  the  archenteron 


ec 


Fig.    27. — Diagrams   Illustrating   the   Formation   of  the   Gastral 

Mesoderm  in   the  Gecko. 

ce.    Chorda    endoderm ;    ec,    ectoderm ;    en,    secondary   endoderm ;    ^w^ 

primary  endoderm;   gm,   gastral  mesoderm. —  (Will.) 


as  in  the  lower  forms.  All  the  prostomial  mesoderm  does 
not,  however,  arise  from  the  primitive  streak  region,  but 
a  considerable  amount  also  has  its  origin  from  the  ectoderm 
covering  the  yolk  outside  the  limits  of  the  blastoderm  proper, 
a  mode  of  origin  which  serves  to  explain  the  phenomena 
later  to  be  described  for  the  mammalia. 

In  comparison  with  the  amphibians  and  Ainphioxus,  the 
reptilia  present  a  subordination  of  the  process  of  invagina- 
tion in  the  formation  of  the  endoderm,  a  primary  endoderm 


56 


FORMATION    OF    THE    GERM    LAYERS. 


making  its  appearance  independently  of  an  invagination, 
and,  in  association  with  this  subordination,  there  is  an  early 
appearance  of  the  primitive  streak,  which,   from  analogy 


Fig.  28. — Sections  of  Ova  of  a  Bat  showing   (A)   the  Formation 

OF    THE    EnDODERM    AND     {B    AND    C)     OF    THE    AMNIOTIC    CaVITY. — 

(Van  Bcneden.) 

with  what  occurs  in  the  amphibia,  may  be  assumed  to  rep- 
resent a  portion  of  the  blastopore  which  is  closed  from  the 
very  beginning. 


FORMATION    OF    THE    GERM    LAYERS.  57 

Turning  now  to  the  mammalia,  it  will  be  found  that  these 
peculiarities  become  still  more  emphasized.  The  inner  cell- 
mass  of  these  forms  corresponds  to  the  blastoderm  of  the 
reptilian  ovum,  and  the  first  differentiation  which  appears 
in  it  concerns  the  cells  situated  next  the  cavity  of  the  vesicle, 
these  cells  dift'erentiating  to  form  a  distinct  layer  which 
gradually  extends  so  as  to  form  a  complete  lining  to  the 
inner  surface  of  the  enveloping  cells  (Fig.  28,  A).  The 
layer  so  formed  is  endodermal  and  corresponds  to  the  pri- 
mary endoderm  of  the  reptiles. 

Before  the  extension  of  the  endoderm  is  completed,  how- 
ever, cavities  begin  to  appear  in  the  cells  constituting  the 
remainder  of  the  inner  mass,  especially  in  those  immediately 
beneath  Rauber's  cells  (Fig.  28,  B),  and  these  cavities  in 
time  coalesce  to  form  a  single  large  cavity  bounded  above 
by  cells  of  the  enveloping  layer  and  below  by  a  thick  plate 
of  cells,  the  einhryonic  disk  (Fig.  28,  C).  The  cavity  so 
formed  is  the  ainuiotic  cavity,  whose  further  history  will  be 
considered  in  a  subsequent  chapter. 

It  may  be  stated  that  this  cavity  varies  greatly  in  its  devel- 
opment in  different  mammals,  being  entirely  absent  in  the 
rabbit  at  this  stage  of  development  and  reaching  an  excessive 
development  in  such  forms  as  the  rat,  mouse,  and  guinea-pig. 
The  condition  here  described  is  that  which  occurs  in  the  bat 
and  the  mole,  and  it  seems  probable,  from  what  occurs  in  the 
youngest  human  embryos  hitherto  observed,  that  the  processes 
in  man  are  closely  similar. 

While  these  changes  have  been  taking  place  a  splitting 
of  the  enveloping  layer  has  occurred,  so  that  the  wall  of 
the  ovum  is  now  formed  of  three  la3^ers,  an  outer  one  which 
may  be  termed  the  trophodcrin,  a  middle  one  which  prob- 
ably is  transformed  into  the  extra-embr3'onic  mesoderm  of 
later  stages,  though  its  sigmificance  is  at  present  somewhat 
obscure,  and  an  inner  one  which  is  the  primary  endoderm. 
In  the  bat,  of  whose  ovum  Fig.  28,  C,  represents  a  section, 


58 


FORMATION    OF    THE    GERM    LAYERS. 


that  portion  of  the  middle  layer  which  forms  the  roof  of 
the  amniotic  cavity  disappears,  only  the  trophoderm  per- 
sisting in  this  region,  but  in  another  form  this  is  not  the 
case,  the  roof  of  the  cavity  being  composed  of  both  the 
trophoderm  and  the  middle  layer. 

A  rabbit's  ovum  in  which  there  is  yet  no  amniotic  cavity 
and  no  splitting  of  the  enveloping  layer  shows,  when  viewed 
from  above,  a  relatively  small  dark  area  on  the  surface. 


-7n^ 


Fig.  29. — A,  Side  View  of  Ovum  of  Rabbit  Seven  Days  Old 
(Kolliker)  ;  B,  Embryonic  Disk  of  a  jNIole  (Heape)  ;  C,  Embry- 
onic Disk  of  a  Dog's  Ovum  of  about  Fifteen  Days   (Bonnet). 

ed,  Embryonic  disk ;  hn,  Hensen's  node ;  mg,  medullary  groove ;  ps, 
primitive   streak;   va,   vascular   area. 

which  is  the  embryonic  disk.  But  if  it  be  looked  at  from 
the  side  (Fig.  29,  A),  it  will  be  seen  that  the  upper  half 
of  the  ovum,  that  half  in  which  the  embryonic  disk  occurs, 
is  somewhat  darker  than  the  lower  half,  the  line  of  sepa- 
ration of  the  two  shades  corresponding  with  the  edge  of  the 
primary  endoderm  which  has  extended  so  far  in  its  growth 


FORMATION    OF    THE    GERM    LAYERS.  59 

around  the  inner  surface  of  the  enveloping  layer.  A  little 
later  a  dark  area  appears  at  one  end  of  the  embryonic  disk, 
produced  by  a  proliferation  of  cells  in  this  region  and  having 
a  somewhat  crescentic  form.  As  the  embryonic  disk  in- 
creases in  size  a  longitudinal  band  makes  its  appearance, 
extending  forward  in  the  median  line  nearly  to  the  center 
of  the  disk,  and  represents  the  primitive  streak  (Fig.  29,  B), 
a  slight  groove  along  its  median  line  forming  what  is  termed 
the  primitive  groove.  In  slightly  later  stages  an  especially 
dark  spot  may  be  seen  at  the  front  end  of  the  primitive 
streak  and  is  termed  Hensen's  node  (Fig.  29,  C,  Jm),  while 


Fig.    30. — Posterior    Portion    of   a    Longitudinal    Section    through 

THE  Embryonic  Disk  of  a  Mole. 
hi,   Blastopore,   ec,   ectoderm ;    en,   endoderm ;    prm,   prostomial   meso- 
derm.—  {After   Hcapc.) 

still  later  a  dark  streak  may  be  observed  extending  forward 
from  this  in  the  median  line  and  is  termed  the  licad-process 
of  the  primitive  streak. 

To  understand  the  meaning  of  these  various  dark  areas 
recourse  must  be  had  to  the  study  of  sections.  A  longi- 
tudinal section  through  the  embryonic  disk  of  a  mole  ovum 
at  the  time  when  the  crescentic  area  makes  its  appearance 
is  shown  in  Fig.  30.  Here  there  is  to  be  seen  near  the 
hinder  edge  of  the  disk  what  is  potentially  an  opening  {hi), 
in  front  of  which  the  ectoderm  {ec)  and  primary  endoderm 
{en)  can  be  clearly  distinguished,  while  behind  it  no  such 
distinction  of  the  two  layers  is  visible.  This  stage  may  be 
regarded  as  comparable  to  a  stage  immediately  preceding 
the  invagination  stage  of  the  reptilian  ovum,  and  the  region 


6o  FORMATION    OF    THE    GERM    LAYERS. 

behind  the  blastopore  will  correspond  to  the  reptilian  primi- 
tive streak.  The  later  forward  extension  of  the  primitive 
streak  is  due  to  the  mode  of  growth  of  the  embryonic  disk. 
Between  the  stages  represented  in  Figs.  30  and  29,  B,  the 
disk  has  enlarged  considerably  and  the  primitive  streak  has 
shared  in  its  elongation.  Since  the  blastopore  of  the  earlier 
stage  is  situated  immediately  in  front  of  the  anterior  ex- 
tremity of  the  primitive  streak,  the  point  corresponding  to 
it  in  the  older  disk  is  occupied  by  Hensen's  node,  this  struc- 
ture, therefore,  representing  a  proliferation  of  cells  from 
the  region  formerly  occupied  by  the  blastopore. 

As  regards  the  head  process,  it  is  at  first  a  solid  cord  of 
cells  which  grows  forward  in  the  median  line  from  Hansen's 
node,  lying  between  the  ectoderm  and  the  primary  endo- 
derm.  Later  a  lumen  appears  in  the  center  of  the  cord, 
forming  what  has  been  termed  the  chorda  canal,  and,  in 
some  forms,  including  man,  the  canal  opens  to  the  surface 
at  the  center  of  Hensen's  node.  The  cord  then  fuses  with 
the  subjacent  primary  endoderm  and  then  opens  out  along 
the  line  of  fusion,  becoming  thus  transformed  into  a  flat 
plate  of  cells  continuous  at  either  side  with  the  primary 
endoderm  (Fig.  31,  Clip).  The  portion  of  the  chorda  canal 
which  traverses  Hensen's  node  now^  opens  below  into  what 
will  be  the  primitive  digestive  tract  and  is  termed  the  nciu  en- 
teric canal  (Fig.  32,  nc)  ;  it  eventually  closes  completely, 
being  merely  a  transitory  structure.  The  similarity  of  the 
head  process  to  the  invagination  which  in  the  reptilia  forms 
the  secondary  endoderm  seems  clear,  the  onl}'  essential  dif- 
ference being  that  in  the  mammalia  the  head  process  arises 
as  a  solid  cord  which  subsequently  becomes  hollow,  instead 
of  as  an  actual  invagination.  The  difference  accounts  for 
the  occurrence  of  Hensen's  node  and  also  for  the  mode  of 
formation  of  the  neurenteric  canal,  and  cannot  l^e  considered 
as  of  great  moment  since   the   development  of   what   are 


FORMATION    OF    THE    GERM    LAYERS. 


6l 


eventually  tubular  structures  (e.  g.,  glands)  as  solid  cords 
of  cells  which  subsequently  hollow  out  is  of  common  occur- 
rence in  the  mammalia.  It  should  be  stated  that  in  some 
mammals  apparently  the  most  anterior  portion  of  the  roof 


itt^ 


LCtfi' 


• 

M^ 

:       4 

CAjk 


Fig.  31. — Transverse  Section  of  the  Embryonic   Area  of  a   Dog's 

Ovum  at  about  the  Stage  of  Development  shown  in  Fig.  29,  C. 
The  section  passes  through  the  head  process   {Clip)  ;  M,  mesoderm. — ■ 

{Bomiet.) 

of  the  archenteron  is  formed  directly  from  the  cells  of  the 
primary  endoderm,  which  in  this  region  are  not  replaced 
by  the  head  process,  but  aggregate  to  form  a  compact  plate 
of  cells  with  which  the  anterior  extremity  of  the  head  proc- 


FiG.   32. — Diagram   of   a   Longitudinal   Section   through   the   Em- 
bryonic Disk  of  a  Mole. 
am,  Amnion ;  ce,  chorda  endoderm ;  ec,  ectoderm ;  nc,  neurenteric  canal ; 
ps,    primitive    streak. —  (Heape.) 

ess  unites.     Such  a  condition  would  represent  a   further 
modification  of  the  original  condition. 

As  regards  the  formation  of  the  mesoderm  it  is  possible 
to  recognize  both  the  prostomial  and  gastral  mesoderm  in 


62 


FORMATION    OF    THE    GERM    LAYERS. 


the  mammalian  ovum,  though  the  two  parts  are  not  so 
clearly  distinguishable  as  in  lower  forms.  A  mass  of  pro- 
stomial  mesoderm  is  formed  from  the  primitive  streak,  and 
when  the  head  process  grows  forward  it  carries  with  it  some 
of  this  tissue.  But,  in  addition  to  this,  a  contribution  to 
the  mesoderm  is  also  apparently  furnished  by  the  cells  of 
the  head  process,  in  the  form  of  lateral  plates  situated  on 
each  side  of  the  middle  line.  These  plates  are  at  first  solid 
(Fig.  33,  gill),  but  their  cells  cjuickly  arrange  themselves  in 
two  layers,  between  which  a  coelomic  space  later  appears. 


Fig.   32- — Transverse  Section  through  the  Embryonic  Disk   of  a 

Rabbit. 
chj  Chorda  endoderm;  ee,  ectoderm;  en^  endoderm ;  gm,  gastral  meso- 
derm.—  (After  van  Beneden.) 


Furthermore,  as  has  already  been  pointed  out,  the  layer 
of  enveloping  cells  splits  into  two  concentric  layers,  the 
inner  of  which  seems  to  be  mesodermal  in  its  nature  and 
forms  a  layer  lining  the  interior  of  the  trophoderm  and 
lying  between  this  and  the  primary  endoderm.  This  layer 
is  by  no  means  so  evident  in  the  lower  forms,  but  is  perhaps 
represented  in  the  reptilian  ovum  by  the  cells  which  underlie 
the  ectoderm  in  the  regions  peripheral  to  the  blastoderm 
proper  (see  p.  55). 

It  has  been  experimentally  determined  (Assheton,  Peebles) 
that  in  the  chick,  whose  embryonic  disk  presents  many  features 
similar  to  those  of  the  mammalian  ovum,  the  central  point  of 
tbe  unincubated  disk  corresponds  to  the  anterior  end  of  the 
primitive  streak  and  to  the  point  situated  immediately  behind 


SIGNIFICANCE    OF    THE    GERM    LAYERS. 


63 


the  heart  of  the  later  embryo  and  immediately  in  front  of  the 
first  mesodermic  somite  (see  p.  103),  as  shown  in  Fig.  34.  If 
these  results  be  regarded  as  applicable  to  the  human  embryo, 
then  it  may  be  supposed  that  in  this  the  head  region  is  devel- 
oped from  the  portion  of  the  embryonic  disk  situated  in  front 


B 


D 


Fig.  34. — Diagrams  Illustrating  the  Relations  of  the  Chick 
Embryo  to  the  Primitive  Streak  at  Different  Stages  of  De- 
velopment.— (Peebles.) 

of  Hensen's  node,  while  the  entire  trunk  is  a  product  of  the 
region  occupied  by  the  node. 

The  Significance  of  the  Germ  Layers. — The  formation 
of  the  three  germ  layers  is  a  process  of  fundamental  impor- 
tance, since  it  is  a  differentiation  of  the* cell  units  of  the 
ovum  into  tissues  which  have  definite  tasks  to  fulfil.  As 
has  been  seen,  the  first  stage  in  the  development  of  the 
layers  is  the  formation  of  the  ectoderm  and  endoderm,  or, 
if  the  physiological  nature  of  the  layers  be  considered,  it 
is  the  differentiation  of  a  layer,  the  endoderm,  which  has 


64  SIGNIFICANCE    OF    THE    GERM    LAYERS. 

principally  nutritive  functions.  In  certain  of  the  lower 
in^'ertebrates.  the  class  Ccelentera,  the  differentiation  does 
not  proceed  beyond  this  diploblastic  stage,  but  in  all  higher 
forms  the  intermediate  layer  is  also  developed,  and  with  its 
appearance  a  further  division  of  the  functions  of  the  organ- 
ism supervenes,  the  ectoderm,  situated  upon  the  outside  of 
the  body,  assuming  the  relational  functions,  the  endoderm 
becoming'  still  more  exclusively  nutritive,  while  the  remain- 
ing functions,  supportive,  excretory,  locomotor,  reproduc- 
tive, etc.,  are  assumed  by  the  mesoderm. 

The  manifold  adaptations  of  development  obscure  in  cer- 
tain cases  the  fundamental  relations  of  the  three  layers, 
certain  portions  of  the  mesoderm,  for  instance,  failing  to 
differentiate  simultaneously  with  the  rest  of  the  layer  and 
appearing  therefore  to  be  a  portion  of  either  the  ectoderm 
or  endoderm.  But,  as  a  rule,  the  layers  are  structural  units 
of  a  hig-her  order  than  the  cells,  and  since  each  assumes 
definite  physiological  functions,  definite  structures  have  their 
origin  from  each. 

Thus  from  the  ectoderm  there  develop : 

1.  The  epidermis  and  its  appendages,  hairs,  nails,  epi- 
dermal glands,  and  the  enamel  of  the  teeth. 

2.  The  epithelium  lining  the  mouth  and  the  nasal  cavities, 
as  well  as  that  lining  the  lower  part  of  the  rectum. 

3.  The  ner^'Ous  system  and  the  nervous  elements  of  the 
sense-organs,  together  with  the  lens  of  the  eye. 

From  the  endoderm  develop : 

1.  The  epithelium  lining  the  digestive  tract  in  general, 
together  with  that  of  the  various  glands  associated  with  it, 
such  as  the  liver  and  pancreas. 

2.  The  lining  epithelium  of  the  larynx,  trachea,  and  lungs. 

3.  The  epithelium  of  the  bladder  and  urethra  (in  part). 
From  the  mesoderm  there  are  formed : 

I.  The' various  connective  tissues,  including  bone  and  the 
teeth  (except  the  enamel). 


LITERATURE.  65 

2.  The  muscles,  both  striated  and  non-striated. 

3.  The  circulatory  system,  including  the  blood  itself  and 
the  lymphatic  system. 

4.  The  lining  membrane  of  the  serous  cavities  of  the 
body. 

5.  The  kidneys  and  ureters. 

6.  The  internal  organs  of  reproduction. 

From  this  list  it  will  be  seen  that  the  products  of  the 
mesoderm  are  more  varied  than  those  of  either  of  the  other 
layers.  Among  its  products  are  organs  in  which  in  either 
the  embryonic  or  adult  condition  the  cells  are  arranged  in 
a  definite  layer,  while  in  other  structures  its  cells  are  scat- 
tered in  a  matrix  of  non-cellular  material,  as,  for  example, 
in  the  connective  tissue,  bone,  cartilage,  and  the  blood  and 
lymph.  It  has  been  proposed  to  distinguish  these  two  forms 
of  mesoderm  as  mcsofhelium  and  mesenchyme  respectively, 
a  distinction  which  is  undoubtedly  convenient,  though  prob- 
ably devoid  of  the  fundamental  importance  which  has  been 
attributed  to  it  by  some  embryologists. 

LITERATURE. 

R.  AsSHETON  :  "  The  Reinvestigation  into  the  Early  Stages  of  the  De- 
velopment of  the  Rabbit,"  Quarterly  Jo  urn.  of  Microsc.  Science, 
XXXVII,  1894. 

R.  AssHETON :  "  The  Development  of  the  Pig  During  the  First  Ten 
Days,"  Quarterly  Journ.  of  Microsc.  Science,  xli,  1898. 

R.  AssHETON :  "  The  Segmentation  of  the  Ovum  of  the  Sheep,  with 
Observations  on  the  Hypothesis  of  a  Hypoblastic  Origin  for  the 
Trophoblast,"  Quarterly  Journ.  of  Microsc.  Science,  xli,  1898. 

E.  VAN  Beneden  :  "  Recherches  sur  les  premiers  stades  du  developpe- 
ment  du  Murin  (Vespertilio  murinus),"  Anatom.  Anseiger,  xvi, 
1899. 

R.  Bonnet  :  "  Beitrage  zur  Embryologie  der  Wiederkauer  gewonnen  am 
Schafei,"  ArcJiiv  fiir  Anat.  nnd  Physiol,  Anat.  Abtli.,  1884  and 
1889. 

R.  Bonnet:  "Beitrage  zur  Embrj^ologie  des  Hundes,"  Anat.  Hcfte.  ix, 
1897. 
7 


66  LITERATURE. 

G.  Born  :  "  Erste  Entwickelungsvorgange,"  Ergcbnisse  der  Anat.  und 
Ent-djicklungsgesch.,  i,  1892. 

E.  G.  CoNKLiN :  "  The  Cause  of  Inverse  Symmetry,"  Anatom.  Anzeigcr, 

XXIII,  1903. 

A.  C.  Eycleshymer:   "The  Early  Development  of  Amblystoma  with 

Observations  on  Some  Other  Vertebrates,"  Journ.  of  Morphol.,  x, 
1895. 

B.  Hatschek  :  "  Studien  iiber  Entwicklung  des  Amphioxus,"  Arbcifen 

aiis  dein  zoolog.  Instit.  zu  Wien,  iv,  1881. 

W.  Heape:  "The  Development  of  the  Mole  (Talpa  europgea),"  Quar- 
terly Journ.  of  Microsc.  Science,  xxiii,  1883. 

A.  A.  W.  HuBRECHT :  "  Studies  on  Mammalian  Embryology  II :  The 
Development  of  the  Germinal  Layers  of  Sorex  vulgaris,"  Quar- 
terly Journ.  of  Microsc.  Science,  xxxi,  1890. 

F.  Keibel  :  "  Studien  zur  Entwickkmgsgeschichte  des  Schweines,"  Mor- 

pholog.  Arhciten,  iii,  1893. 
M.   Kunsemuller:   "Die   Eifurchung  des   Igels    (Erinaceus   europseus 

L.),"  Zcitschr.  fiir  wissensch.  Zool.,  lxxxv,  1906. 
K.  MiTSUKURi  and  C.  Ishikawa  :  "  On  the  Formation  of  the  Germinal 

Layers  in  Chelonia,"  Quarterly  Journ.  of  Microsc.  Science,  xxvii, 

1887. 
F.    Peebles  :    "  The   Location   of  the   Chick   embryo  upon  the   Blasto- 
derm," Journ.  of  E.rper.  Zool.,  1,  1904. 
E.  Selenka  :  "  Studien  iiber  Entwickelungsgeschichte  der  Thiere,"  4tes 

Heft,  1886-87;  5tes  Heft,  1891^2. 
J.    SoEOTTA :    "  Die   Befruchtung  und   Furchung  des   Eies   der   Maus," 

Archiv  fiir  mikrosk.  Anat.,  xisv,  1895. 
J.  SoBOTTA :  "  Die  Furchung  des  Wirbelthiereies,"  Ergcbnisse  der  Anat. 

und  Entzvickelungsgeschichte,  vi,  1897. 
J.   SoBOTTA :   "  Neuere  Auschauungen  iiber  die  Entstehung  der  Doppel 

(miss)    bildungen,    mit   besonderer   Beriicksichtigung   der   mensch- 

lichen   Zwillingsgeburten,"   Wurzburgcr  Abhandl.,  1,   1901. 
H.  H.  Wilder:  "Duplicate  Twins  and  Double  Monsters,"  Amcr.  Jour. 

of  Anat.,  Ill,  1904. 
L.  Will  :  "  Beitrage  zur  Entwicklungsgeschichte  der  Reptilien,"  Zoolog. 

Jahrbilchcr,  Abth.  fiir  Anat.,  vi,  1893. 


CHAPTER  III. 

THE  DEVELOPMENT  OF  THE  EXTERNAL  FORM 
OF  THE  HUMAN  EMBRYO. 

The  youngest  human  ovum   at  present  known  is   that 
described  by  Peters.     It  was  taken  from  the  uterus  of  a 


CbTTV 


Fig.  35. — Section  of  Embryo  and  Adjacent  Portion  of  an  Ovum  of 

I     MM. 

am,  Amniotic  cavity ;  ce,  chorionic  ectoderm ;  cm,  chorionic  mesoderm ; 
ec,  embryonic  ectoderm;  en,  endoderm;  m,  embryonic  mesoderm; 
ys,  yol]<-sac. —  {Peters.') 


woman  who  had  committed  suicide  one  calendar  month 
after  the  last  menstruation,  and  it  measured  about  i  mm. 
in  diameter.  The  entire  inner  surface  of  the  trophoderm 
(Fig.  35,  ce')  was  lined  by  a  layer  of  mesoderm  (cm),  which, 

67 


68  DEVELOPMENT    OF    EXTERNAL    FORM. 

on  the  surface  furthest  away  from  the  uterine  cavity,  was 
considerably  thicker  than  elsewhere,  forming  an  area  of 
attachment  of  the  embryo  to  the  wall  of  the  ovum.  In 
the  substance  of  this  thickening  was  the  amniotic  cavity 
(am),  whose  roof  was  formed  by  flattened  cells,  which,  at 
the  sides,  became  continuous  with  a  layer  of  columnar  cells 
forming  the  floor  of  the  cavity  and  constituting  the  em- 
bryonic ectoderm  (ec).    Immediately  below  this  was  a  layer 


Fig.  36. — Diagrams  to  show  the  Probable  Relationships  of  the 
Parts   in   the   Embryos   Represented   in   Figs.    28,    C,   and   35. 

Ac,  Amniotic  cavity;  C,  extra-embryonic  body-cavity;  Me,  (in  figure  to 
the  left)  mesoderm,  (in  figure  to  the  right)  somatic  mesoderm; 
Me',  splanchnic  mesoderm ;  D,  digestive  tract ;  En,  endoderm ;  T, 
trophoblast.  The  broken  line  in  the  mesoderm  of  the  figure  to 
the  left  indicates  the  line  along  which  the  splitting  of  the  meso- 
derm occurs. 

of  mesoderm  (m)  which  split  at  the  edge  of  the  embryonic 
disk  into  two  layers,  one  of  which  became  continuous  with 
the  mesodermic  thickening  and  so  with  the  layer  of  meso- 
derm lining  the  interior  of  the  trophoderm,  while  the  other 
enclosed  a  sac  lined  by  a  layer  of  endodermal  cells  and 
termed  the  yolk-sac  (ys).  The  total  length  of  the  embryo 
was  0.19  mm.,  and  so  far  as  its  ectoderm  and  mesoderm  are 
concerned  it  might  be  described  as  a  flat  disk  resting  on  the 
surface  of  the  yolk-sac,  thoug"h  it  must  he  understood  that 


-DEVELOPMENT    OF    EXTERNAL    FORM.  69 

the  yolk-sac  also  to  a  certain  extent  forms  part  of  the 
embryo. 

This  embryo  seems  to.be  in  an  early  stage  of  the  primi- 
tive streak  formation,  before  the  development  of  the  head 
process.  On  comparing  it  with  the  ovum  of  a  bat  in  ap- 
proximately the  stage  of  development  represented  in  Fig. 
28,  C,  it  will  be  seen  to  present  some  important  advances 
(Fig.  36).  It  seems  clear  that  the  yolk-sac  is  equivalent 
to  what  was  the  cavity  of  the  ovum  in  the  earlier  stages, 
and  consequently  the  cavity  (c)  into  which  the  yolk-sac 
projects  is  unrepresented  in  the  bat's  ovum.  How  this 
cavity  is  formed  can  only  be  conjectured,  but  it  seems 
probable  that  it  arises  by  the  splitting  of  the  layer  of  cells 
which  lines  the  interior  of  the  trophoderm  in  the  bat's  ovum 
(or  perhaps  by  the  vacuolization  of  the  central  cells  of  this 
layer)  and  the  subsequent  accumulation  of  fluid  between 
the  two  mesodermal  layers  so  formed.  However  that  may 
be,  it  seems  clear  that  the  size  of  the  human  ovum  is  due 
mainly  to  the  rapid  growth  of  this  cavity,  which,  as  future 
stages  show,  is  the  extra-embryonic  portion  of  the  body- 
cavity,  the  splitting  or  vacuolization  of  the  mesoderm  by 
which  it  is  probably  formed  being  the  precocious  appear- 
ance of  the  typical  splitting  of  the  mesoderm  to  form  the 
embryonic  body-cavity  which,  as  will  be  seen  in  a  subse- 
quent chapter,  takes  place  only  at  a  later  stage  of  develop- 
ment. From  now  on  the  trophoderm  and  the  layer  of  me- 
soderm lining  it  may  together  be  spoken  of  as  the  chorion, 
the  mesoderm  layer  being  termed  the  chorionic  mesoderm. 

A  human  embryo  of  a  somewhat  greater  age  (Fig.  37), 
measuring  about  0.37  mm.  in  length,  has  been  described 
by  Graf  Spec  as  embryo  v.Yi.,  and  was  taken  from  an  ovum 
estimated  to  measure  6  by  4.5  mm.  in  diameter.  Notwith- 
standing the  much  greater  size  of  the  ovum,  which  is  due 
to  the  continued  increase  in  the  size  of  the  extra-embryonic 


JO 


DEVELOPMENT    OF   EXTERNAL   FORM. 


coelom,  the  embryo  is  but  little  advanced  beyond  the  stage 
which  the  Peters'  embryo  had  reached,  and  is  probably  in  a 
late  stage  of  the  development  of  the  primitive  streak.  Con- 
fining the  attention  for  the  present  solely  to  the  embryo  and 
the  immediately  adjoining  parts,  it  will  be  seen  that  the 
thickening  of  the  chorionic  mesoderm  which  encloses  the 
amniotic  cavity  has  increased  in  size  and  now  forms  a  ped- 
icle, known  as  the  helly-stalk  (b),  at  the  extremity  of  which 
is  the  yolk-sac  (3;).     Furthermore,  the  amniotic  cavity  (a) 


Fig.  37. — Ovum  Measuring  6  X  Fig.    38. — Embryo    1.54    mm.    in 

4.5    mm.    The   Left    Half   of  Length,     from     the     Dorsal 

THE    Chorion    has    Been    Re-  Surface. 

moved  to  show  the  Embryo.  a.  Amnion ;  m,  medullary  groove ; 

a,  Amniotic  cavity;  b,  belly-stalk;  nc,  neurenteric  canal;  ps,  primi- 

c,  chorion;    e,   embryonic   disk;  tive    streak;    y,   yolk-sac. — {von 

V,  chorionic  villus;   y,  yolk-sac.  Spee.) 

—  {von  Spee.) 


now  lies  somewhat  excentrically  in  this  pedicle,  being  near 
what  may  be  spoken  of  as  its  anterior  surface.  The  em- 
bryo still  possesses  a  discoidal  form  and  may  still  be  de- 
scribed as  a  flat  disk  floating  on  the  surface  of  the  yolk-sac. 
This  same  general  form  is  preserved  in  another  embryo, 
known  as  embryo  Gle^  described  by  Graf  Spee,  which  meas- 
ured 1.54  mm.  in  length  (Fig.  38).  In  it,  however,  the 
more  median  portion  of  the  embryonic  disk  has  become 


-DEVELOPMENT    OF    EXTERNAL    FORM. 


71 


thicker  and  is  separated  from  the  more  peripheral  portions 
by  a  distinct  furrow.  From  the  more  median  or  axial  por- 
tion the  embryo  proper  will  develop,  and  this  portion  is  now 
shaped  somewhat  like  the  body  of  a  violin  and  presents  at 
its  posterior  portion  the  remains  of  the  primitive  streak, 
near  the  anterior  end  of  which  is  a  distinct  pore,  the  open- 
ing of  what  is  termed  the  neurenteric  canal  {nc),  whose 


Fig.  39. — Diagrams  Illustrating  the  Constriction  of  the  Embryo 

FROM  THE  Yolk-sac. 

A  and  C  are  longitudinal,  and  B  and  D  transverse  sections.     B  is  drawn 

to  a  larger  scale  than  the  other  figures. 


significance  has  already  been  discussed  (p.  60).  More 
anteriorly  two  longitudinal  ridges  have  appeared,  the  first 
indications  of  which  are  termed  the  medullary  folds. 

In  later  stages  a  separation  or  constriction  of  the  embryo 
from  the  yolk-sac  begins  and  results  in  the  transformation 
of  the  discoidal  embryonic  portion  of  the  embryonic  disk 
into  a  cylindrical  structure.  Primarily  this  depends  upon 
the  deepening  of  the  furrow  which  surrounds  the  embryonic 
area,  the  edges  of  this  area  being  thus  bent  in  on  all  sides 


72  DEVELOPMENT    OF    EXTERNAL    FORM. 

toward  the  yolk-sac.  This  bending  in  proceeds  most  rap- 
idly at  the  anterior  end  of  the  body,  as  shown  in  the  dia- 
grams (Fig.  39),  and  least  rapidly  at  the  posterior  end 
where  the  belly-stalk  is  situated,  and  produces  a  constriction 
of  the  yolk-sac,  the  portion  of  that  structure  nearest  the 
embryonic  disk  becoming  enclosed  within  the  body  of  the 
embryo  to  form  the  digestive  tract,  while  the  remainder  is 
converted  into  a  pedicle-like  portion,  the  yolk-stalk,  at  the 

extremity  of  which  is  the 
yolk-vesicle.  The  further  con- 
tinuance of  the  folding  in  of 
the  edges  of  the  embryonic 
area  leads  to  an  almost  com- 
plete closing  in  of  the  di- 
gestive tract  and  reduces  the 

T-,  T-,  T  opening    through    which    the 

biG.   40. — Embryo  2.5   mm.   Long.      j-  cs  & 

am,    Fragment    of    the    torn    am-  yolk-stalk      and      belly-Stalk 

nion;    mg,    medullary    groove;  communicate   with    the    em- 
Y,     yolk-sac. —  {Allen     Thomp-  .         . 

son.)  bryonic   tissues    to   a    small 

area  known  as  the  umhilicus. 
An  embryo  which  exhibits  an  early  stag'e  in  the  process 
of  constriction  has  been  described  by  Allen  Thompson  and 
is  represented  in  Fig.  40.*  It  measured  about  2.5  mm.  in 
length  and  had  reached  a  stage  in  which  the  medullary 
folds  had  become  very  pronounced  and  their  edges  had 
come  into  contact  at  one  portion,  although  the  anterior  and 
posterior  portions  of  the  groove  {fng)  between  them  were 
still  widely  open.  The  embryo  will  be  seen  from  the  figure 
to  project  somewhat  both  in  front  of  and  behind  the  yolk- 
sac,  although  the  greater  part  of  its  ventral  surface  is  still 
formed  by  that  structure.  At  the  sides  also  it  is  well  sepa- 
rated from  the  yolk-sac,  and  resting  upon  the  sac  in  front 
is  a  swelling  which  represents  the  heart. 

*  It  must  be  noted  that  m  the  figure  neither  the  amnion  (except  for  a 
small  fragment  still  persisting  in  front)  nor  the  belly-stalk  is  represented. 


DEVELOPMENT    OF    EXTERNAL    FORM. 


73 


In  another  embryo  (Fig.  41),  slightly  smaller  though 
evidently  older  than  the  preceding  one,  and  described  by 
Eternod,  the  edges  of  the  medullary  folds  have  not  only 
come  into  contact  throughout  the  greater  portion  of  their 
length,  but  they  have  fused  together,  the  groove  between 
them  being  open  only  in  front  and  behind.  On  each  side 
of  the  median  line  eight  somewhat  oblong  areas  are  to  be 


Fig.  41. — Reconstruction  of  Embryo  2.1  i  mm.  Long. 
al,  Allantois;   am,  amnion;  B,  belly-stalk;  ch,  chorion;  h,  heart;   ms, 
mesodermic  somite;  os,  oral  fossa;  ph,  pharynx;  v,  chorionic  villi; 
Y,  yolk-sac. — (After  Eternod.) 


distinguished,  caused  by  a  transverse  division  of  the  sub- 
jacent mesoderm  into  what  are  termed  mesodermic  somites 
(ms),  structures  which  will  be  described  in  detail  in  the 
succeeding  chapter.  The  separation  of  the  embryo  from 
the  yolk-sac  (F)  has  advanced  considerably  and  the  sac 
shows  evident  indications  of  constriction  just  where  it  meets 


74 


DEVELOPMENT    OF    EXTERNAL    FORM. 


the  body  of  the  embryo.     The  head  projects  more  markedly 
beyond  the  anterior  surface  of  the  yolk-sac  and  is  separated 


0^' 


Fig.  42. — Embryo  2.5   mm.   Long. 
am,  Amnion ;   B,  belly-stalk ;   h,  heart ;  M,  closed,  and  M',  still  open 
portions  of  the  medullary  groove ;   Om,  omphalo-mesenteric  vein ; 
OS,  oral  fossa;   Y,  yolk-sac. — (Kollmann.) 


from  the  region  occupied  by  the  heart  (h)  by  a  deep  and 
well-marked  depression,  the  oral  fossa  (os). 

In   an   embryo   described  1)y   Kollmann    (Fig.   42)    and 


-DEVELOPMENT    OF    EXTERNAL    FORM. 


75 


measuring  2.5  mm.  in  length,*  the  edges  of  the  medullary 
folds  (M)  had  come  into  contact  throughout  their  entire 
length,  except  for  a  short  distance  anteriorly  (M^),  and 
thirteen  mesodermic  somites  were  visible.  The  constric- 
tion of  the  yolk-sac  was  even  more  pronounced  than  in 
the  preceding  embryo  and  the  hind  end  of  the  body  had 


(.<;}, 


(/vT^ 


yjcv^ 


am-. 


wsP^^ 


Fig.   43. — Embryo   Lg,   2.15    mm.   Long. 
am,  Amnion;  B,  belly-stalk;  C,  chorion;  ]i,  heart;  Y,  yolk-sac. — (His.) 

become  defined,  the  belly-stalk  no  longer  seeming  to  be  a 
posterior  continuation  of  the  body  but  arising  from  the 
posterior  part  of  the  ventral  surface.  The  oral  fossa  (OS) 
was  also   more  marked,   and   it   may  be  noticed   that   the 

*  The  embryo  was  measured  only  after  having  been  preserved  in 
alcohol,  and  the  actual  length  was  probably  somewhat  greater  than  this. 


J^  DEVELOPMENT    OF    EXTERNAL    FORM. 

dorsal  surface  of  the  body  was  distinctly  concave  from 
before  backward,  a  peculiarity  which  becomes  more  pro- 
nounced in  a  later  stage  and  constitutes  what  is  termed 
the  dorsal  flexure. 

This  is  well  shown  in  an  embryo  described  by  His  and 
named  by  him  embryo  lxviii  (Lg)  (Fig.  43).  In  it  the 
yolk-sac  forms  a  much  smaller  portion  of  the  ventral  sur- 
face than  it  did  in  earlier  stages,  and  it  has  also  become 
distinctly  separated  from  the  belly-stalk.  The  most  pecu- 
liar feature  of  this  embryo  is,  however,  the  dorsal  flexure. 
This  is  apparently  a  normal  feature  and  is  probably  pro- 
duced by  a  difference  in  the  rate  of  growth  of  the  lateral 
and  median  portions  of  the  outer  layer  of  the  embryonic 
mesoderm,  the  former  portion  failing  to  keep  pace  with 
the  growth  of  the  latter,  which  becomes  folded  in  accom- 
modation to  the  strain.  The  flexure  is  of  comparatively 
short  duration,  and  when  once  it  begins  to  disappear  it 
seems  to  do  so  rapidly,  the  dorsal  concavity  suddenly  be- 
coming a  convexity  and  the  tension  of  the  layer  coming 
into  equilibrium  in  the  new  position.  One  other  feature 
is  noteworthy  in  this  embryo — namely,  the  occurrence  of 
two  linear  vertical  depressions  a  little  behind  the  head 
region  of  the  embryo;  these  are  the  first  representatives  of 
a  series  of  branchial  clefts. 

These  structures  are  of  great  morphological  importance, 
inasmuch  as  they  determine  to  a  large  extent  the  arrange- 
ment of  various  organs  of  the  head  region.  They  repre- 
sent the  clefts  which  exist  in  the  walls  of  the  pharynx  in 
fishes,  through  which  water,  taken  in  at  the  mouth,  passes 
to  the  exterior,  bathing  on  its  way  the  gill  filaments  attached 
to  the  bars  or  arches,  as  they  are  termed,  which  separate 
successive  clefts.  Hence  the  name  "  branchial  "  which  is 
applied  to  them,  though  in  the  mammals  they  never  have 
respiratory    functions   to   perform,   but,   appearing,   persist 


DEVELOPMENT    OF    EXTERNAL    FORM. 


T7 


for  a  time  and  then  either  disappear  or  are  apphed  to  some 
entirely  different  purpose.  Indeed,  in  man  they  are  never 
really  clefts  but  merely  grooves,  and  corresponding  to  each 
groove  in  the  ectoderm  there  is  also  one  in  the  subjacent 
endoderm  of  what  will  eventually  be  the  pharyngeal  region 
of  the  digestive  tract,  so  that  in  the  region  of  each  cleft  the 
ectoderm  and  endoderm  are  in  close  relation,  being  sepa- 
rated only  by  a  very  thin 
layer  of  mesoderm,  while  in 
the  intervals  between  suc- 
cessive clefts  a  more  con- 
siderable amount  of  meso- 
derm is  present  (Fig.  44). 
In  the  human  embryo 
four  clefts  develop  on  each 
side  of  the  body  and  five 
branchial  arches,  the  last 
arch  lying  posteriorly  to 
the  fourth  cleft  and  not 
being  very  sharply  defined 
along  its  posterior  margin. 

As  just  stated,  the  clefts  are  normally  merely  grooves,  and 
in  later  development  either  disappear  or  are  converted  into 
special  structures.  Occasionally,  however,  a  cleft  may  persist 
and  the  thin  membrane  which  forms  its  floor  may  become  per- 
forated so  that  an  opening  from  the  exterior  into  the  pharynx 
occurs  at  the  side  of  the  neck,  forming  what  is  termed  a 
branchial  fistula.  Such  an  abnormality  is  most  frequently  de- 
veloped from  the  lower  (ventral)  part  of  the  first  cleft;  nor- 
mally this  disappears,  the  upper  portion  persisting,  however, 
to  form  the  external  auditory  meatus  and  tympanic  cavity. 

The  embryo  lxviii  (Lg)  just  described  measured  2. 11 
mm.  in  length,  this  measurement,  however,  being  taken 
along  a  straight  line  and  not  following  the  flexure  of  the 
body.  It  does  not,  therefore,  represent  the  actual  length 
of  the  body  and  there  is  much  less  difference  between  this 


Fig.  44. — Floor  of  the  Pharynx 
OF   Embryo   B,  7   mm.  Long. 

Ep,  Epiglottis ;  Sp,  sinus  prsecervi- 
calis ;  f,  anterior,  and  f,  pos- 
terior portions  of  the  tongue ;  I, 
II,  III,  and  /F,  branchial  arches. 
—  (His.) 


78 


DEVELOPMENT    OF    EXTERNAL    FORM. 


YiG   4=;.— Embryo  Lr,  4-2  mm.  Long. 

„„,  A„,„lo„;  »..,  auditory  cap,„le ;  Sbeliy-s.a|k;»    heart;  H  lower 

and  Ul,  upper  limb;  F,  yolk-sac.     <^ini.; 


DEVELOPMENT    OF    EXTERNAL    FORM.  79 

and  the  next  to  be  described  than  is  impHed  by  the  figures. 
This  embryo  (Fig.  45)  is  also  one  of  those  described  by 
His  and  is  known  as  embryo  lxvii  (Lr).  It  measures  4.2 
mm.  in  length  and  shows  an  almost  complete  disappearance 
of  the  dorsal  flexure  so  marked  in  embryo  lxviii.  Instead 
of  this,  it  presents  a  well-marked  ventral  bending  of  both 
the  anterior  and  posterior  portions  of  the  body,  so  that  the 
dorsal  surface  is  prominently  curved  in  the  regions  which 
will  later  be  the  nape  of  the  neck  and  the  sacral  region,  and 
consequently  the  convexities  may  be  known  as  the  neck  bend 
and  the  sacral  bend.  Furthermore,  there  is  noticeable  a 
ventral  projection  of  the  extreme  front  end  of  the  body,  so 
that  a  third  convexity  occurs  anterior  to  the  neck  bend  and 
may  be  termed  the  head  bend. 

The  constriction  of  the  yolk-sac  has  progressed ;  the  meso- 
dermic  somites  have  almost  reached  their  maximum  devel- 
opment and  are  very  distinct ;  the  two  branchial  clefts  pres- 
ent in  the  preceding  embryo  have  increased  in  size  and  the 
third  cleft  has  made  its  appearance ;  two  small  elevations  of 
the  sides  of  the  body,  one  almost  opposite  the  neck  bend 
and  the  other  opposite  the  sacral  bend,  are  the  first  indica- 
tions of  the  limbs  (Ul  and  LI)  ;  and  the  eyeball  and  ear 
vesicle  (an),  which  were  present  though  hot  very  evident 
in  earlier  stages,  are  now  plainly  visible  in  surface  views. 

In  the  next  stage — as  a  type  of  which  an  embryo  figured 
by  Coste  (Fig.  46)  may  be  taken — the  three  bends  of  the 
body  mentioned  above  have  greatly  increased,  so  that  the 
head  and  tail  of  the  embryo  are  almost  in  contact  and  the 
latter  is  bent  a  little  toward  one  side.  The  closure  of  the 
ventral  surface  of  the  body  is  almost  completed  and  the 
margins  of  the  umbilicus  have  begun  to  be  prolonged  ven- 
trally  so  as  to  enclose  the  yolk-stalk  and  belly-stalk  in  the 
umbilical  cord.  The  yolk-sac  has  increased  considerably  in 
length  and  the  differentiation  of  its  extra-embryonic  por- 


8o 


DEVELOPMENT    OF    EXTERNAL    FORM. 


Fig.   46. — Embryo   of   from   Twenty   to   Twenty-five   Days. 

Im,  Amnion;   LL,  lower  limb;    UA,  umbilical  artery;    Uc,  umbilical 

cord;    UL,  upper   limb;    Ys,  yolk-sac. —  (Coste.) 


DEVELOPMENT    OF    EXTERNAL    FORM.  <5 1 

tions  into  a  yolk-stalk  and  yolk-vesicle  is  plainly  distin- 
guishable. The  limb  rudiments  have  increased  somewhat 
in  size,  and,  in  addition  to  the  eyeball  and  ear  vesicle,  a 
third  sense-organ  has  made  its  appearance  in  the  form  of 
two  pits  situated  on  the  under  side  of  the  anterior  portion 
of  the  head;  these  pits  are  the  first  indications  of  the  nasal 
fosscF. 

The  fourth  branchial  cleft  has  appeared  and  those  formed 


^J7f 


—LI 


Fig.  47. — Embryo  9.1   mm.  Long. 
U,  Lower  limb;  JJ ,  umbilical  cord;  Ul,  upper  limb;  Y ,  yolk-sac. —  (His.) 

earlier  have  elongated  so  that  they  almost  reach  the  mid- 
ventral  line,  and  from  the  dorsal  part  of  the  anterior  border 
of  the  first  arch  a  strong  process  has  developed  so  that  the 
arch  on  each  side  is  somewhat  < -shaped.  The  upper  limb 
of  each  V  is  destined  to  give  rise  to  the  upper  jaw,  and 


82 


DEVELOPMENT    OF    EXTERNAL    FORM. 


hence  is  known  as  the  maxillary  process,  while  the  lower 
limb  represents  the  lower  jaw  and  is  termed  the  mandibular 
process. 

Leaving  aside  for  the  present  all  consideration  of  the 
further  development  of  the  limbs  and  branchial  arches,  the 
further  evolution  of  the  general  form  of  the  body  may  be 
rapidly  sketched.  In  an  embryo  (Fig.  47)  from  Ruge's 
collection,    described   and   figured   by   His   and   measuring 


Fig.   48. — Embryo  Br:;,   13.6   mm.   Long. —  (His.) 

9.1  mm.  in  length,*  the  prolongation  of  the  margins  of  the 
umbilicus  has  increased  until  more  than  half  the  yolk-stalk 
has  become  enclosed  within  the  umbilical  cord.  The  neck 
and  sacral  bends  are  still  very  pronounced,  although  the 
embryo  is  beginning-  to  straighten  out  and  is  not  quite  so 
much  coiled  as  in  the  preceding  stage.     At  the  posterior 

*  This  measurement  is  taken  in  a  straight  Hne  from  the  most  anterior 
portion  of  the  neck  bend  to  the  middle  point  of  the  sacral  bend  and 
does  not  follow  the  curvature  of  the  embryo.  It  may  be  spoken  of  as 
the  neck-rump  length  and  is  convenient  for  use  during  the  stages  when 
the  embryo  is  coiled  upon  itself. 


DEVELOPMENT    OF    EXTERNAL    FORM. 


83 


end  of  the  body  there  has  developed  a  rather  abruptly  con- 
ical tail  filament,  in  the  place  of  the  blunt  and  gradually 
tapering  termination  seen  in  earlier  stages,  and  a  well- 
marked  rotundity  of  the  abdomen,  due  to  the  rapidly  in- 
creasing size  of  the  liver,  begins  to  become  evident. 

In  later  stages  the  enclosure  of  the  yolk-  and  belly-stalks 
within  the  umbilical  cord  proceeds  until  finally  the  cord  is 
complete  through  the  entire  interval  between  the  embryo 


I        '■- 


Fig.   49. — A,    Embryo    S2,    15    mm.   Long    (showing    Ectopia   of   the 
Heart)  ;   B,  Embryo  L3,   17.5  mm.  Long.— (-f/j^.) 

and  the  wall  of  the  ovum.  At  the  same  time  the  straight- 
ening out  of  the  embryo  continues,  as  may  be  seen  in  Fig. 
48  representing  the  embryo  xlv  (Bra)  of  His,  which  shows 
also,  both  in  front  of  and  behind  the  neck  bend,  a  distinct 
depression,  the  more  anterior  being  the  occipital  and  the 
more  posterior  the  neck  depression ;  both  these  depressions 
are  the  expressions  of  changes  taking  place  in  the  central 
nervous    system.     The    tail    filament    has    become    more 


84  DEVELOPMENT    OF    EXTERNAL    FORM. 

marked,  and  in  the  head  region  a  sHght  ridge  surrounding 
the  eyeball  and  marking  out  the  conjunctival  area  has 
appeared;  a  depression  anterior  to  the  nasal  fossae  marks 
off  the  nose  from  the  forehead ;  and  the  external  ear,  whose 
development  will  be  considered  later  on,  has  become  quite 
distinct.     This  embryo  had  a  neck-rump  length  of  13.6  mm. 

In  the  embryos  xxxv  (Sg)  and  xcix  (L3)  (Fig.  49,  A 
and  B)  of  His'  collection  the  straightening  out  of  the  neck 
bend  is  proceeding,  and  indeed  is  almost  completed  in  em- 
bryo xcix,  which  begins  to  resemble  closely  the  fully  formed 
fetus.  The  tail  filament,  somewhat  reduced  in  size,  still 
persists  and  the  rotundity  of  the  abdomen  continues  to  be 
well  marked.  The  neck  region  is  beginning  to  be  distin- 
guishable in  embryo  xxxv  and  in  embryo  xcix  the  eyelids 
have  appeared  as  slight  folds  surrounding  the  conjunctival 
area.  The  nose  and  forehead  are  clearly  defined  by  the 
greater  development  of  the  nasal  groove  and  the  nose  has 
also  become  raised  above  the  general  surface  of  the  face, 
while  the  external  ear  has  almost  acquired  its  final  fetal 
form.  These  embryos  measure  respectively  about  15  and 
17.5  mm.  in  length.* 

Finally,  an  embryo — again  one  of  those  described  by 
His,  namely,  his  lxxvii  (Wt),  having  a  length  of  23  mm. — 
may  be  figured  (Fig.  50)  as  representing  the  practical 
acquisition  of  the  fetal  form.  This  embryo  dates  from 
about  the  end  of  the  second  month  of  pregnancy,  and  from 
this  period  onward  it  is  proper  to  use  the  term  fetus  rather 
than  that  of  embryo.  The  changes  which  have  been  de- 
scribed in  preceding  stages  are  now  complete  and  it  remains 
only  to  be  mentioned  that  the  caudal  filament,  which  is  still 
prominent,  gradually  disappears  in  later  stages,  becoming, 
as  it  were,  submerged  and  concealed  beneath  adjacent  parts 

*The  embryo  xxxv  presents  a  slight  abnormality  in  the  great  pro- 
jection of  the  heart,  but  otherwise  it  appears  to  be  normal. 


DEVELOPMENT    OF    EXTERNAL    FORM. 


85 


by  the  development  of  the  buttocks.  The  incompleteness 
of  the  development  of  these  regions  in  embryo  lxxvii  is 
manifest,  not  only  from  the  projection  of  the  tail  filament, 
but  also  from  the  external  genitalia  being  still  largely  visible 


Fig.  50. — Embryo  Wt,  23  mm.  Long. —  (His.) 

in  a  side  view  of  the  embryo,  a  condition  which  will  dis- 
appear in  later  stages. 

The  Later  Development  of  the  Branchial  Arches,  and 
the  Development  of  the  Face. — In  Coste's  embryo  (Fig. 
46)  the  four  branchial  clefts  and  five  arches  which  develop 
in  the  human  embryo  are  visible  in  surface  views,  but  in  the 
Ruge  embryo  (Fig.  47)  it  will  be  noticed  that  only  the  first 


86 


DEVELOPMENT    OF    THE    BRANCHIAL    ARCHES. 


two  arches,  the  first  \Yith  a  weh-developed  -maxillary  proc- 
ess, and  the  cleft  separating  them  can  be  distinguished. 
This  is  due  to  a  sinking  inward  of  the  region  occupied  by 
the  three  posterior  arches  so  that  a  triangiilar  depression, 
the  sinus  prccccrvicalis,  is  formed  on  each  side  of  what  will 
later  become  the  anterior  part  of  the  neck  region.  This  is 
well  shown  in  an  embryo  (Br3)  described  by  His  which 
measured  6.9  mm.  in  length  and  of  which  the  anterior  por- 
tion is  shown  in  Fig.  51.     The  anterior  boundary  of  the 


na 


/ 


Fig.  51. — Head  of  Embryo  of  6.9  mm. 
na.  Nasal  pit;  ps,  precervical  sinus. —  (His.) 

sinus  (ps)  is  formed  by  the  posterior  edge  of  the  second 
arch  and  its  posterior  boundary  by  the  thoracic  wall,  and 
in  later  stages  these  two  boundaries  gradually  approach  one 
another  so  as  first  of  all  to  diminish  the  opening  into  the 
sinus  and  later  to  completely  obliterate  it  by  fusing  together, 
the  sinus  thus  l)ecoming  converted  into  a  completely  closed 
cavity  whose  lloor  is  formed  by  the  ectoderm  covering  the 


DEVELOPMENT    OF    THE    BRANCHIAL    ARCHES.  8/ 

three  posterior  arches  and  the  clefts  separating  these.  This 
cavity  eventually  undergoes  degeneration,  no  traces  of  it 
occurring  normally  in  the  adult,  although  certain  cysts  occa- 
sionally observed  in  the  sides  of  the  neck  may  represent 
persisting  portions  of  it. 

A  somewhat  similar  process  results  in  the  closure  of  the 
ventral  portion  of  the  first  cleft,*  a  fold  growing  backward 
from  the  posterior  edge  of  the  first  arch  and  fusing  with 
the  ventral  part  of  the  anterior  border  of  the  second  arch. 


Fig.  52. — Face  of  Embryo  of  8  mm. 

mxp,   Maxillary  process ;   nl^,  nasal  pit ;   os,  oral   fossa ;   pg,  processus 

globularis. — (His.) 

The  upper  part  of  the  cleft  persists,  however,  and,  as 
already  stated,  forms  the  external  auditory  meatus,  the 
pinna  of  the  ear  being  developed  from  the  adjacent  parts 
of  the  first  and  second  arches  (Figs.  48  and  49). 

The  region  immediately  in  front  of  the  first  arch  is  occu- 
pied by  a  rather  deep  depression,   the  oral   fossa,  whose 

*  See  page  77,  small  type. 


88  DEVELOPMENT    OF    THE    FACE. 

early  development  has  already  been  traced.  In  an  embryo 
measuring  8  mm.  in  length  (Fig.  52)  the  fossa  (os)  has 
assumed  a  somewhat  irregular  quadrilateral  form.  Its  pos- 
terior boundary  is  formed  by  the  mandibular  processes  of 
the  first  arch,  while  laterally  it  is  bounded  by  the  maxillary 
processes  (mxp)  and  anteriorly  by  the  free  edge  of  a 
median  plate,  termed  the  nasal  process^  which  on  either  side 


Fig.    53. — Face   of    Embryo    after   the    Completion    of    the    Upper 

Jaw. —  (His.) 


of  the  median  line  is  elevated  to  form  a  marked  protuber- 
ance, the  proccssits  glohularis  (pg).  The  ventral  ends  of 
the  maxillary  processes  are  widely  separated,  the  nasal 
process  and  the  processus  glojjulares  intervening  between 
them,  and  they  are  also  separated  from  the  globular  proc- 


-     DEVELOPMENT    OF    THE    LIMBS.  89 

esses  by  a  deep  and  rather  wide  groove  which  anteriorly 
opens  into  a  circular  depression,  the  nasal  pit   (np). 

Later  on  the  maxillary  and  globular  processes  unite, 
obliterating  the  groove  and  cutting  off  the  nasal  pits — 
which  have  by  this  time  deepened  to  form  the  nasal  fossre 
— from  direct  communication  with  the  mouth,  with  which, 
however,  they  later  make  new  communications  behind  the 
maxillary  processes,  an  indication  of  the  anterior  and  pos- 
terior nares  being  thus  produced. 

Occasionally  the  maxillary  and  globular  processes  fail  to 
unite  on  one  or  both  sides,  producing  a  condition  popularly 
known  as  "  harelip." 

At  the  time  when  this  fusion  occurs  the  nasal  fossae  are 
widely  separated  by  the  broad  nasal  process  (Fig.  53),  but 
during  later  development  this  process  narrows  to  form  the 
nasal  septum  and  is  gradually  elevated  above  the  general 
surface  of  the  face  as  shown  in  Figs.  48-50.  By  the  nar- 
rowing of  the  nasal  process  the  globular  processes  are 
brought  nearer  tog'ether  and  form  the  portions  of  the  upper 
jaw  immediately  on  each  side  of  the  median  line,  the  rest 
of  the  jaw  being  formed  by  the  maxillary  processes.  In 
the  meantime  a  furrow  has  appeared  upon  the  mandibular 
process,  running  parallel  with  its  borders  (Fig.  49)  ;  the 
portion  of  the  process  in  front  of  this  furrow  gives  rise  to 
the  lower  lip  and  is  known  as  the  lip  ridge,  while  the  portion 
behind  the  furrow  becomes  the  lower  jaw^  proper  and  is 
termed  the  chin-  ridge. 

The  Development  of  the  Limbs. — As  has  been  already 
pointed  out,  the  limbs  make  their  appearance  in  an  embryo 
measuring  about  4  mm.  in  length  (Fig.  45)  and  are  at  first 
bud-like  in  form.  As  they  increase  in  length  they  at  first 
have  their  long  axes  directed  parallel  to  the  longitudinal 
axis  of  the  body  and  become  somewhat  flattened  at  their 
free  ends,  remaining  cylindrical  in  their  proximal  portions. 
9 


90  DEVELOPMENT    OF    THE   LIMBS. 

A  furrow  or  constriction  appears  at  the  junction  of  the 
flattened  and  cylindrical  portions  (Fig.  47),  and  later  a 
second  constriction  divides  the  cylindrical  portion  into  a 
proximal  and  distal  moiety,  the  three  segments  of  each  limb 
— the  arm,  forearm,  and  hand  in  the  upper  limb,  and  the 
thigh,  leg,  and  foot  in  the  lower — being  thus  marked  out. 
The  digits  are  first  indicated  by  the  development  of  four 
radiating  shallow  grooves  upon  the  hand  and  foot  regions, 
and  a  transverse  furrow  uniting  the  proximal  ends  of  the 
digital  furrows  indicates  the  junction  of  the  digital  and 
palmar  regions  of  the  hand  or  of  the  toes  and  body  of  the 
foot.  After  this  stage  is  reached  the  development  of  the 
upper  limb  proceeds  more  rapidly  than  that  of  the  lower, 
although  the  processes  are  essentially  the  same  in  both  limbs. 
The  digits  begin  to  project  slightly,  but  are  at  first  to  a  very 
considerable  extent  united  together  by  a  web,  whose  further 
growth,  however,  does  not  keep  pace  with  that  of  the  digits, 
which  thus  come  to  project  more  and  more  in  later  stages. 
Even  in  comparatively  early  stages  the  thumb,  and  to  a 
somewhat  slighter  extent  the  great  toe,  is  widely  separated 
from  the  second  digit  (Figs.  49  and  50). 

While  these  changes  have  been  taking  place  the  entire 
limbs  have  altered  their  position  with  reference  to  the  axis 
of  the  body,  being  in  stages  later  than  that  shown  in  Fig. 
47  directed  ventrally  so  that  their  longitudinal  axes  are  at 
right  angles  to  that  of  the  body.  From  the  figures  of  later 
stages  it  may  be  seen  that  it  is  the  thumb  (radial)  side  of 
the  arm  and  the  great  toe  (tibial)  side  of  the  leg  which  are 
directed  forward;  the  plantar  and  palmar  surfaces  of  the 
feet  and  hands  are  turned  toward  the  body  and  the  elbow 
is  directed  outward  and  slightly  backward,  while  the  knee 
looks  outward  and  slightly  forward.  It  seems  proper  to 
conclude  that  the  radial  side  of  the  arm  is  homologous  with 
the  tibial  side  of  the  leg,  the  palmar  surface  of  the  hand 


-    DEVELOPMENT    OF    THE    LIMBS.  9 1 

with  the  plantar  surface  of  the  foot,  and  the  elbow  with 
the  knee. 

The  limbs  are,  however,  still  in  the  quadrupedal  condi- 
tion, and  they  must  later  undergo  a  second  alteration  in 
position  so  that  their  long  axes  again  become  parallel  with 
that  of  the  body.  This  is  accomplished  by  a  rotation  of 
the  limbs  around  axes  passing  through  the  shoulders  and 
hip- joints  together  with  a  rotation  about  their  longitudi- 
nal axes  through  an  angle  of  90  degrees.  This  axial  rota- 
tion of  the  upper  limb  is,  however,  in  exactly  the  opposite 
direction  to  that  of  the  lower  limb  of  the  corresponding 
side,  so  that  the  homologous  surfaces  of  the  two  limbs  have 
entirely  different  relations,  the  radial  side  of  the  arm,  for 
instance,  being  the  outer  side  while  the  tibial  side  of  the  leg 
is  the  inner  side,  and  whereas  the  palmar  surface  of  the 
hand  looks  ventrally,  the  plantar  surface  of  the  foot  looks 
dorsally. 

In  making  these  statements  no  account  is  taken  of  the 
secondary  position  which  the  hand  may  assume  as  the  result 
of  its  pronation;  the  positions  given  are  those  assumed  by 
the  limbs  when  both  the  bones  of  their  middle  segment  are 
parallel  to  one  another. 

It  may  he  pointed  out. that  the  prevalent  use  of  the  physio- 
logical terms  flexor  and  extensor  to  describe  the  surfaces  of 
the  limbs  has  a  tendency  to  obscure  their  true  morphological 
relationships.  Thus  if,  as  is  usual,  the  dorsal  surface  of  the 
arm  be  termed  its  extensor  surface,  then  the  same  term  should 
be  applied  to  the  entire  ventral  surface  of  the  leg,  and  all  move- 
ments of  the  lower  limb  ventrally  should  be  spoken  of  as 
movements  of  extension  and  any  movement  dorsally  as  move- 
ments of  flexion.  And  yet  a  ventral  movement  of  the  thigh 
is  generally  spoken  of  as  a  flexion  of  the  hip- joint,  while  a 
straightening  out  of  the  foot  upon  the  leg — that  is  to  say,  a 
movement  of  it  dorsally — is  termed  its  extension. 

The  Age  of  the  Embryo  at  Different  Stages. — The  age 

of  an  embryo  must  be  dated  from  the  moment  of  fertiliza- 


92  AGE    OF    EMBRYO    AT    DIFFERENT    STAGES. 

tion  and  from  what  has  been  said  in  previous  pages  (pp..  35, 
36)  it  is  evident  that  it  must  be  exceedingly  difficult  to 
determine  the  exact  age  of  any  embryo  even  when  the  time 
of  the  cessation  of  the  menses  and  the  date  of  the  coition 
which  resulted  in  the  pregnancy  are  known.  And,  further- 
more, not  only  is  the  actual  date  of  the  beginning  of  devel- 
opment uncertain,  but  in  the  majority  of  the  known  human 
embryos  in  early  stages  the  time  of  the  cessation  of  develop- 
ment is  also  more  or  less  uncertain,  since  the  embryos  are 
abortions  and  their  expulsion  need  not  necessarily  have  im- 
mediately succeeded  their  death. 

These  various  sources  of  uncertainty  are  of  especial  im- 
portance in  the  early  stages  of  development,  when  a  day 
more  or  less  means  much.  But  nevertheless  it  is  conve- 
nient to  have  some  estimate  of  the  age  of  such  embr5^os 
even  though  it  be  recognized  that  any  date  given  is  a  mere 
approximation.  His  has  made  an  estimate  of  the  age  of  a 
number  of  embryos  concerning  which  approximate  data 
were  available  with  results  which  are  stated  in  the  follow- 
ing table : 

At  2-2*  weeks  the  embryo  measures  2.2-  3      mm.  in  length. 

"    2i-3        "  "  "  3    -  4-S  mm- 

"     3i         "  "  "5-6     mm.  " 

"       4         "  "  "7-8     mm. 

"      4*  "  "  "         10    -II      mm.  " 

5  "  "  "         13  mm. 

It  must  be  borne  in  mind,  however,  that  embiyos  of  the 
same  age  need  not  in  all  cases  be  of  the  same  length,  since 
conditions  of  nutrition,  etc.,  will  largely  determine  not  only 
the  size  of  the  embryo,  but  also  the  amount  of  its  develop- 
ment. And,  furthermore,  it  seems  probable  that  the  esti- 
mates for  age  given  in  the  above  table  may  be  too  small, 
since  there  is  reason  to  believe  that  the  earlier  stages  of 
development  proceed  more  slowly  than  do  the  later  ones. 


AGE    OF    EMBRYO    AT    DIFFERENT    STAGES.  93 

Thus,  Bischoff  found  that  the  embryonic  disk  in  the  rabbit 
showed  but  Httle  differentiation  up  to  the  seventh  or  eighth 
day,  while  at  the  tenth  day  the  embryo  possessed  branchial 
clefts  and  mesodermic  somites.  It  would  seem  from  the 
available  data,  which  are  more  definite  than  usual,  that  a 
human  embryo  described  by  Eternod  and  measuring  only 
1.3  mm.  in  length  w^as  very  nearly  twenty-one  days  old; 
and  if  this  estimate  be  correct  then  the  ages  assigned  by  His 
to  the  earlier  embryos  must  be  very  considerably  increased. 
As  regards  the  later  periods  of  development,  the  limits 
of  error  for  any  date  become  of  less  importance.  His  esti- 
mates that  at  the  end  of  the  second  month  when  the  embryo 
becomes  a  fetus,  its  length  is  about  25  to  28  mm.,  and  for 
later  periods  Schroder  gives  the  following  measurements  as 
the  averag'e : 

3d  lunar  month,  ^o-  90  mm. 

4th  "  " 100-170  mm. 

5th  "  "  180-270  mm. 

6th  "  "  280-340  mm. 

7th  "  "  350-380  mm. 

8th  "  " 425  mm. 

9th  "  "  467  mm. 

loth  "  "  490-500  mm. 

From  the  study  of  a  relatively  large  number  of  embryos 
Mall  concludes  that  the  ages  of  embryos  measuring  any- 
where from  I  to  100  mm.  in  length  may  be  expressed  in 
days  with  a  fair  amount  of  accuracy  by  the  square  root  of 
the  length  multiplied  by  100  (V  length  in  mm.  X  100), 
and  that  in  embryos  between  100  and  220  mm.  the  age  in 
days  is  about  the  same  as  the  length  in  millimeters. 

The  data  concerning  the  weight  of  embryos  of  different 
ages  are  as  yet  very  insufficient,  and  it  is  well  known  that 
the  weights  of  new-born  children  may  vary  greatly,  the 
authenticated  extremes  being,  according  to  Vierordt,  717 
grams  and  6123  grams.     It  is  probable  that  considerable 


94  LITERATURE. 

variations  in  weight  occur  also  during  fetal  life.  So  far 
as  embryos  of  the  first  two  months  are  concerned,  the  data 
are  too  imperfect  for  tabulation ;  for  later  periods  Fehling 
gives  the  following  as  average  weights : 

3d  month,  20  grams. 

4th  "  120 

5th  "  28s 

6th  " 635 

7th  "  1220 

8th  "  1700 

9th  "  2240 

loth  "  3250 

LITERATURE. 

J.  Broman  :  "  Beobachtung  eines  menschhchen  Embryos  von  beinahe  3 
mm.  Lange  mit  specieller  Bemerkung  iiber  die  bei  demselben  befind- 
Itchen  Hirnfalten',"  Morpholog.  Arbeitcn,  v,  1895. 

J.  M.  CoSTE :  "  Histoire  generale  et  particuhere  du  developpement  des 
corps  organises,"   Paris,   1847-1859. 

A.  EcKER :  "  Beitrage  zur  Kenntniss  der  ausserer  Formen  jiingster 
menschlichen  Embryonen,"  Archk'  fiir  Aiiat.  und  Physiol.,  Anat. 
Ahth.,  1880. 

A.  C.  F.  Eternod  :  "Communication  sur  im  oeuf  humain  avec  embryon 
excessivement  jemie,"  Archives  Ital.   dc   Biologic,   xxii,   1895. 

A.  C.  F.  Eternod  :  "  II  y  a  nn  canal  notochordal  dans  1'  embryon  hu- 
main," Anat.  Anzcigcr,  xvi,  1899. 

C.  GiAcoMiNi:  "  Un  ceuf  humain  de  11  jours,"  Archives  Ital.  de  Biol- 
ogic, XXIX,  1898. 

V.  Hensen  :  "  Beitrag  zur  Morphologic  der  Korperform  und  des 
Gehirns  des  menschlichen  Embryos,"  Arehiv  fiir  Anat.  und  Physiol, 
Anat.  Abth.,  1877. 

W.   His  :   "  Anatomic  menschlicher  Embryonen,"   Leipzig,   1880. 

J.  Janosi7<:  :  "  Zwei  jungc  mcnschliche  Embryonen,"  Arehiv  far  mikrosk. 
Anat.,  XXX,   1887. 

F.  Keibel:  "  Ein  sehr  junges  menschliches  Ei,"  Arehiv  fiir  Anat.  und 
Physiol,  Anat.  Abth.,  1890. 

F.  Keibel  :  "  Ueber  einen  menschlichen  Embryo  von  6.8  mm.  grosster 
Lange,"  Verhandl.  Anatom.  Gesellseh.,  xiii,  1899. 

J.  KoLLMANN :  "  Die  Korperform  menschtichcr  normaler  und  patho- 
logischer  Embryonen,"  Arehiv  fiir  Anat.  und  Physiol,  Anat  Abth., 
Supplement,  1889. 


LITERATURE.  95 

F.  P.  Mall  :  "  A  Human  Embryo  Twenty-six  Days  Old,"  Journ.  of 
Morphology,  v,  1891. 

F.  P.  Mall:  "A  Human  Embryo  of  the  Second  Week,"  Anat.  Anseiger, 
VIII,  1893. 

F.  P.  Mall  :  "  Early  Human  Embryos  and  the  Mode  of  their  Preserva- 
tion," Bulletin  of  the  Johns  Hopkins  Hospital,  iv,  1894. 

C.  S.  Minot:  "Human  Embryology,"  New  York,   1892. 

J.  Muller  :  "  Zergliederungen  menschlicher  Embryonen  aus  f riiherer 
Zeit,"  Archiv  fiir  Anat.  und  Physiol.,   1830. 

H.  Peters  :  "  Ueber  die  Einbettung  des  menschlichen  Eies  und  das 
friiheste  bisher  bekannte  menschliche  Placentarstadium,"  Leipzig 
und   Wien,    1899. 

C.  Phisalix:  "Etude  d'un  Embryon  humain  de  11  millimetres,"  Ar- 
chives de  soolog.  experimentale  et  generale,  Ser.  2,  vi,  1888. 

H.  Piper  :  "  Ein  menschlicher  Embryo  von  6.8  mm.  Nackenlinie," 
Archiv  fiir  Anat.  und  Physiol.,  Anat.  Abth.,  1898. 

F.  Graf  von  Spee  :  "  Beobachtungen  an  einer  menschlichen  Keimscheibe 
mit  ofifener  Medullarrinne  und  Canalis  neurentericus,"  Archiv  fiir 
Anat.  und  Physiol.,  Anat.  Abth.,  1889. 

F.  Graf  von  Spee  :  "  Ueber  f riihe  Entwickelungsstufen  des  mensch- 
lichen Eies,"  .Archiv  fiir  Anat.  und  Physiol.,  Anat.  Abth.,  1896. 

Allen  Thompson  :  "  Contributions  to  the  History  of  the  Structure  of 
the  Human  Ovum  and  Embryo  before  the  Third  Week  after  Con- 
ception, with  a  Description  of  Some  Early  Ova,"  Edinburgh  Med. 
and  Surg.  Journal,  iii,  1839.  (See  also  Froriep's  Neiie  Notisen, 
XIII,   1840.) 


CHAPTER  IV. 

THE  MEDULLARY  GROOVE,  NOTOCHORD,  AND 
MESODERMIC  SOMITES. 

In  the  youngest  human  embryos  known,  such  as  the 
Peters'  embryo  and  the  youngest  embryo  described  by  Graf 
Spee,  there  is  no  differentiation  of  the  embryonic  disk 
other  than  that  associated  with  the  formation  of  the  primi- 
tive streak.  In  an  embi"yo  described  by  Eternod  and  meas- 
uring 1.3  mm.  in  length  (Eig.  54)  a  median  longitudinal 
groove  (m)  has  made  its  appearance,  marking  out  the  axis 
of  the  disk,  and  forming  what  is  known  as  the  medullary 
groove;  and  in  "the  older  embryo  described  by  Spee  (Eig. 
38)  a  longitudinal  ridge  has  appeared  on  either  side  of  the 
groove,  forming  the  medullary  folds. 

The  two  folds  are  continuous  anteriorly,  but  behind  they 
are  at  first  separate,  the  anterior  portion  of  the  primitive 
streak  lying  between  them.  In  forms,  such  as  the  Reptilia, 
which  possess  a  distinct  blastopore,  this  opening  lies  in  the 
interval  between  the  two,  and  consequently  is  in  the  floor 
of  the  medullary  groove,  and  in  the  mammalia,  even  though 
no  well-defined  blastopore  is  formed,  yet  at  the  time  of  the 
formation  of  the  medullary  fold  an  opening  breaks  through 
at  the  anterior  end  of  the  primitive  streak  and  places  the 
cavity  lying  below  the  endoderm  in  communication  with 
the  space  bounded  by  the  medullary  folds.  The  canal  so 
formed  is  termed  the  neurenteric  canal  (Eig.  55,  nc)  and 
is  so  called  because  it  unites  what  will  later  become  the  cen- 

96 


THE    MEDULLARY    GROOVE, 


97 


Fig.   54. — Embryo    1.34   mm.    Long. 
al,    Allantois ;    am,    amnion ;    bs,    belly-stalk ;    h,    heart ;    m,    medullary 
groove;  nc,  neurenteric  canal;  pc,  caudal  protuberance;  ps,  priml 
tive  streak;  ys,  yolk-stalk. —  (Etcnwd.) 


98 


THE    MEDULLARY    GROOVE. 


tral  canal  of  the  nervous  system  with  the  intestine  (en- 
teron).  The  significance  of  this  canal  has  already  been 
discussed  (p.  60)  ;  it  is  of  very  brief  persistence,  closing  at 
an  early  stage  of  development  so  as  to  leave  no  trace  of  its 
existence. 

As  development  proceeds  the  medullary  folds  increase 


Fig.  55. — Diagram  of  a  Longitudinal  SEcrioN  through  an  Embryo 

OF  1.54  MM. 

al,  Allantois ;   am,  amnion ;   B,  belly-stalk ;   ch,  chorion ;   h,  heart ;  nc, 

neurenteric  canal;   V,  chorionic  villi;   Y,  yolk-sac. — (von  Spee.) 

in  height  and  at  the  same  time  incline  toward  one  another 
(Fig.  40)  so  that  their  edges  finally  come  into  contact  and 
later  fuse,  the  two  ectodermal  layers  forming  the  one  unit- 
ing with  the  corresponding  layers  of  the  other  (Fig.  56). 


THE    MEDULLARY    GROOVE. 


99 


By  this  process  the  medullary  groove  becomes  converted 
into  a  medullary  canal  which  later  becomes  the  central 
canal  of  the  spinal  cord  and  the  ventricles  of  the  brain,  the 
ectodermal  walls  of  the  canal  thickening  to  give  rise  to  the 
central  nervous  system.  The  closure  of  the  groove  does 
not,  however,  take  place  simultaneously  along  its  entire 
length,  but  begins  in  what  corresponds  to  the  neck  region 


Fig.    56. — Diagrams   showing  the  Manner  of  the  Closure  of  the 
Medullary  Groove. 


of  the  adult  (Fig.  41)  and  thence  proceeds  both  anteriorly 
and  posteriorly,  the  extension  of  the  fusion  taking  place 
rather  slowly,  however,  especially  anteriorly,  so  that  an 
anterior  opening  into  the  otherwise  closed  canal  can  be 
distinguished  for  a  considerable  period   (Fig.  42). 

While  these  changes  have  been  taking  place  in  the  ecto- 
derm of  the  median  line  of  the  embryonic  disk,  modifica- 
tions of  the  subjacent  endoderm  have  also  occurred.  This 
endoderm,  it  will  be  remembered,  was  formed  by  the  head 


lOO 


THE    NOTOCHORD. 


process  of  the  primitive  streak,  and  was  a  plate  of  cells  con- 
tinuous at  the  sides  with  the  primary  endoderm  and  extend- 
ing forward  as  far  as  what  will  eventually  be  the  anterior 
part  of  the  pharynx.  Along  the  line  of  its  junction  with 
the  primary  endoderm  it  gives  rise  to  the  plates  of  gastral 
mesoderm  (Fig.  27),  while  the  remainder  of  it  produces 
an  important  embryonic  organ  known  as  the  notochord  or 


Fig.    57. — Transverse    Sections    through    Mole   Embryos,    showing 

THE  Formation  of  the  Notochord. 
ec,  Ectoderm;  en,  endoderm;  m,  mesoderm;  nc,  notochord. —  (Heape.) 

chorda  dorsalis  and  on  this  account  is  sometimes  termed 
the  chorda  endoderm. 

After  the  separation  of  the  plates  of  gastral  mesoderm  the 
chorda  endoderm,  which  is  at  first  a  flat  band,  becomes  some- 
what curved  (Fig.  57,  A),  so  that  it  is  concave  on  its  under 
surface,  and,  the  curvature  increasing,  the  edges  of  the  plate 
come  into  contact  and  finally  fuse  together  (Fig.  57,  B), 
the  edges  of  the  primary  endoderm  at  the  same  time  uniting 
beneath  the  chordal  tube  so  formed,  so  this  layer  becomes 
a  continuous  sheet,  as  it  was  at  its  first  appearance.  The 
lumen  which  is  at  first  present  in  the  chordal  tube  is  soon 
obliterated  l^y  the  enlargement  of  the  cells  which  bound  it, 


THE    NOTOCHORD.  lOI 

and  these  cells  later  undergo  a  peculiar  transformation 
whereby  the  chordal  tube  is  converted  into  a  solid  elastic 
rod  surrounded  by  a  cuticular  sheath  secreted  by  the  cells. 
The  notochord  lies  at  first  immediately  beneath  the  median 
line  of  the  medullary  groove,  between  the  ectoderm  and  the 
endoderm,  and  has  on  either  side  of  it  the  mesodermal 
plates.  It  is  a  temporar)^  structure  of  which  only  rudiments 
persist  in  the  adult  condition  in  man,  but  it  is  a  structure 
characteristic  of  all  vertebrate  embryos  and  persists  to  a 
more  or  less  perfect  extent  in  many  of  the  fishes,  being 
indeed  the  only  axial  skeleton  possessed  by  Amphioxus.  In 
the  higher  vertebrates  it  is  almost  completely  replaced  by 
the  vertebral  column,  which  develops  around  it  in  a  manner 
to  be  described  later. 

Turning  now  to  the  middle  germinal  layer,  it  will  be  found 
that  in  it  also  important  changes  take  place  during  these 
early  stages  of  development.  The  probable  mode  of  devel- 
opment of  the  extra-embryonic  mesoderm  and  body-cavity 
has  already  been  described  (p.  69)  and  attention  may  now 
be  directed  toward  what  occurs  in  the  embryonic  mesoderm. 
In  both  the  Peters  embryo  and  the  embryo  v.H  described 
by  von  Spec  this  portion  of  the  mesoderm  is  represented  by 
a  plate  of  cells  lying  between  the  ectoderm  and  endoderm 
and  becoming-  continuous  at  the  edges  of  the  embryonic 
area  with  both  the  layer  which  surrounds  the  yolk-sac  and, 
through  the  mesoderm  of  the  belly-stalk,  with  the  chorionic 
mesoderm  (Fig.  35).  It  seems  probable,  since  there  is  in 
these  embryos  no  indication  as  yet  of  the  formation  of  the 
chorda  endoderm,  that  this  plate  of  mesoderm  corresponds 
to  the  prostomial  mesoderm  of  lower  forms.  In  older  em- 
bryos, such  as  the  embryo  Gle  of  Graf  Spec  and  the  younger 
embryo  described  by  Eternod  (Fig.  54),  the  mesoderm  no 
longer  forms  a  continuous  sheet  extending  completely  across 
the  embryonic  disk,  but  is  divided  into  two  lateral  plates,  in 


I02  THE    MESODERMIC    SOMITES. 

the  interval  between  which  the  ectoderm  of  the  floor  of  the 
medullary  groove  and  the  chorda  endoderm  are  in  close 
contact  (Fig.  34).  These  lateral  plates  represent  the  gas- 
tral  mesoderm,  whose  origin  has  already  been  described 
(p.  62),  and  which  apparently  supplants  the  original  pros- 
tomial  mesoderm,  whose  fate  in  the  human  embryo  is  at 
present  unknown.  The  changes  which  now  occur  have  not 
as  yet  been  observed  in  the  human  embryo,  though  they 
probably  resemble  those  described  in  other  mammalian  em- 
bryos, and  the  phenomena  which  occur  in  the  sheep  may 
serve  to  illustrate  their  probable  nature. 

The  lateral  plates  increase  in  size  by  the  multiplication 


Fig.   58. — Transverse    Section    through    the    Second    Mesodermic 

Somite  of  a   Sheep   Embryo  3    mm.   Long. 
am.  Amnion ;  en,  endoderm ;  /,  intermediate  cell-mass ;  mg,  medullary 

groove;    ms,   mesodermic   somite;    so,   somatic   and   sp,    splanchnic 

layers  of  the  ventral  mesoderm. —  (Bonnet.) 

of  the  cells  which  compose  them  and,  in  sections,  have  a 
somewhat  triangular  form,  the  portions  nearest  the  median 
line  of  the  embryo  being  much  thicker  than  the  more  lateral 
parts.  In  the  region  which  will  later  become  the  neck  of 
the  embryo  a  longitudinal  groove  appears  upon  the  dorsal 
surface  of  each  plate,  marking  off  the  more  median  thicker 
portion  from  the  lateral  parts,  and  the  median  portions  then 
become  divided  transversely  into  a  number  of  more  or  less 


THE    MESODERMIC    SOMITES.  IO3 

cubical  masses  which  are  termed  the  protovertehrcE  or,  better, 
mesodermic  somites  (Fig.  58,  ww),  structures  whose  appear- 
ance in  surface  views  has  already  been  described  (Fig.  41 
ct  seq.).  The  cells  of  the  somites  and  of  the  lateral  parts, 
which  may  be  termed  the  ventral  mesoderm,  are  at  first 
stellate  in  form,  but  later  become  more  spindle-shaped,  and 
those  near  the  center  of  each  somite  and  those  of  the  ventral 
mesoderm  arrange  themselves  in  regular  layers  so  as  to  en- 
close cavities  which  appear  in  these  regions  (Fig,  58).    The 


«9 


^*'''**--&^  J^^'\  <-''-?rv  ^2>;8|fr^ 


Fig.   59. — Transverse  Section  of  an   Embryo  of  2.5   mm.    (See  Fig. 

42)     SHOWING   ON   EITHER    SIDE   OF   THE    MEDULLARY    CaNAL   A    MeSO- 

DERMic  Somite,  the  Intermediate  Cell-mass,  and  the  Ventral 
Mesoderm. —  (^von  Lenhossek.) 

cavities  of  the  somites  first  formed  become  continuous  with 
the  cavities  contained  between  the  layers  of  the  adjacent 
ventral  mesoderm,  but  this  continuity  eventually  disappears 
and  is  not  developed  in  the  later  formed  somites.  Each 
original  lateral  plate  of  gastral  mesoderm  thus  becomes 
divided  longitudinally  into  three  areas,  a  more  median  area 


I04  THE    MESODERMIC    SOMITES. 

composed  of  mesodermic  somites,  lateral  to  this  a  narrow 
area  underlying  the  original  longitudinal  groove  which  sep- 
arated the  somite  area  from  the  ventral  mesoderm  and 
which  from  its  position  is  termed  the  intermediate  cell  mass 
(Fig.  58,  /),  and,  finally,  the  ventral  mesoderm.  This  last 
portion  is  now  divided  into  two  layers,  the  dorsal  of  which 
is  termed  the  soma  fie  incsodcnii,  while  the  ventral  one  is 
known  as  the  splanchnic  mesoderm  (Fig.  58,  so  and  sp;  and 
Fig.  59),  the  cavity  which  separates  these  two  layers  being 
the  embryonic  body-cavity  or  pleiiroperitoneal  cavity,  which 
will  eventually  give  rise  to  the  pleural,  pericardial  and  peri- 
toneal cavities  of  the  adult  as  well  as  the  cavity  of  each 
tunica  vaginalis  testis. 

Beginning  in  the  neck  region,  the  formation  of  the  meso- 
dermic somites  proceeds  anteriorly  and  posteriorly  until 
finally  there  are  present  in  the  human  embryo  thirty-eight 
pairs  in  the  neck  and  trunk  regions  of  the  body,  and,  in 
addition,  a  certain  number  are  developed  in  what  is  later  the 
occipital  region  of  the  head.  Exactly  how  many  of  these 
occipital  somites  are  developed  is  not  known,  but  in  the  cow 
four  have  been  observed,  and  there  are  reasons  for  believing 
that  the  same  number  occurs  in  the  human  embryo. 

In  the  lower  vertebrates  a  number  of  cavities  arranged  in 
pairs  occur  in  the  more  anterior  portions  of  the  head  and  have 
laeen  homologized  with  mesodermic  somites.  Whether  this 
homology  be  perfectly  correct  or  not,  these  head-cavities,  as 
they  are  termed,  indicate  the  existence  of  a  division  of  the 
head  mesoderm  into  somites,  and  although  practically  nothing 
is  known  as  to  their  existence  in  the  human  embryo,  yet, 
from  the  relations  in  which  they  stand  to  the  cranial  nerves 
and  musculature  in  the  lower  forms,  there  is  reason  to  suppose 
that  they  are  not  entirely  unrepresented. 

The  mesodermic  somites  in  the  earliest  human  embryos 
in  which  they  have  been  observed  contain  a  completely 
closed  cavity,  and  this  is  true  of  the  majority  of  the  somites 


THE    MESODERMIC    SOMITES. 


105 


in  such  a  form  as  the  sheep.  In  the  four  first-formed  som- 
ites in  this  species,  however,  as  Has  already  been  stated,  the 
somite  cavity  is  at  first  continuous  with  the  pleuroperitoneal 
cavity  and  only  later  becomes  separated  from  it,  and  in 
lower  vertebrates  this  continuity  of  the  somite  cavities  with 


.♦•^♦^'■=-— ■ 


<       it    x'^**^  ./.'•/•vVV-.-VT*'".  •'  /' 


— ,  ^;, 


F« 


^.>^\-■^/ 


Fig.  60. — Transverse  Section  of  an  Embryo  of  4.25  mm.  at  the 
Level  of  the  Arm  Rudiment. 

A,  Axial  mesoderm  of  arm;  Am,  amnion;  il,  inner  lamella  of  myo- 
tome; Mj  myotome;  wt?,  splanchnic  mesoderm;  ol,  outer  lamella 
of  myotome ;  Pn,  place  of  origin  of  pronephros ;  6^^  sclerotome ; 
.S"^  defect  in  wall  of  myotome  due  to  separation  of  the  sclerotome ; 
St,  stomach;    Vu,  umbilical   vein. —  (KoUmann.) 

the  general  body-cavity  is  the  rule.  The  somite  cavities 
are  consequently  to  be  regarded  as  portions  of  the  general 
pleuroperitoneal  cavity  which  have  secondarily  been  sepa- 


io6 


THE    MESODERMIC    SOMITES. 


rated  off.  They  are,  however,  of  but  short  duration  and 
early  become  filled  up  by  spindle-shaped  cells  derived  from 
the  walls  of  the  somites,  which  themselves  undergo  a  differ- 
entiation into  distinct  portions.  The  cells  of  that  portion 
of  the  wall  of  each  somite  which  is  opposite  the  notochord 
become  spindle-shaped  and  grow  inward  toward  the  median 
line  to  surround  the  notochord  and  central  nervous  system, 
and  give  rise  eventually  to  the  lateral  half  of  the  body  of  a 
vertebra  and  the  corresponding  portion  of  a  vertebral  arch. 
This  portion  of  the  somite  is  termed  a  sclerotome  (Fig.  60, 
S),  and  the  remainder  forms  a  muscle  plate  or  myotome 
(M)  which  is  destined  to  give  rise  to  a  portion  of  the  volun- 
tary musculature  of  the  body.  The  outer  wall  of  the  somite 
has  been  generally  believed  to  take  part  in  the  formation  of 
the  cutis  layer  of  the  integument  and  hence  has  been  termed 
the  Cittts  plate  or  dermatome,  but  it  seems  probable  that  it 
becomes  entirely  transformed  into  muscular  tissue. 

The  intermediate  cell-mass  in  the  human  embryo,  as  in 
lower  forms,  partakes  of  the  transverse  divisions  which 
separate  the  individual  mesodermic  somites.  From  one  por- 
tion of  the  tissue  in  most  of  the  somites  (Fig.  60,  Pn)  the 
provisional  kidneys  or  Wolffian  bodies  develop,  this  portion 
of  each  mass  being  termed  a  ncphrotome,  while  the  remain- 
ing portion  gives  rise  to  a  mass  of  cells  showing  no  tendency 
to  arrange  themselves  in  definite  layers  and  constituting 
that  form  of  mesoderm  which  has  been  termed  mesenchyme 
(see  p.  65).  These  mesenchymatous  masses  become  con- 
verted into  connective  tissues  and  blood-vessels. 

The  ventral  mesoderm  in  the  neck  and  trunk  regions 
never  become  divided  transversely  into  segments  correspond- 
ing to  the  mesodermic  somites,  differing  in  this  respect  from 
the  other  portions  of  the  gastral  mesoderm.  In  the  head, 
however,  that  portion  of  the  middle  layer  which  corresponds 
to  the  ventral  mesoderm  of  the  trunk  does  undergo  a  division 


THE    MESODERMIC    SOMITES.  lO/ 

into  segments  in  connection  with  the  development  of  the 
branchial  arches  and  clefts.  A  consideration  of  these  seg- 
ments, which  are  known  as  the  hranchiomeres,  may  conve- 
niently be  postponed  until  the  chapters  dealing  with  the 
development  of  the  cranial  muscles  and  nerves,  and  in  what 
follows  here  attention  will  be  confined  to  what  occurs  in  the 
ventral  mesoderm  of  the  neck  and  trunk. 

Its  splanchnic  layer  applies  itself  closely  to  the  endo- 
dermal  digestive  tract  (Fig.  6i,  Sp),  which  is  constricted 
off  from  the  dorsal  portion  of  the  yolk-sac,  and  becomes 
converted  into  mesenchyme  out  of  which  the  muscular  coats 
of  the  digestive  tract  develop.  The  cells  which  line  the 
pleuroperitoneal  cavity,  however,  retain  their  arrangement 
in  a  layer  and  form  a  part  of  the  serous  lining  of  the  peri- 
toneal and  other  serous  cavities,  the  remainder  of  the  lining 
being  formed  by  the  corresponding  cells  of  the  somatic 
layer;  and  in  the  abdominal  region  the  superficial  cells,  sit- 
uated near  the  line  where  the  splanchnic  layer  passes  into 
the  somatic,  and  in  close  proximity  to  the  nephrotome  of  the 
intermediate  cell-mass,  become  columnar  in  shape  and  are 
converted  into  reproductive  cells. 

The  somatic  layer,  if  traced  peripherally,  becomes  con- 
tinuous at  the  sides  with  the  layer  of  mesoderm  which  lines 
the  outer  surface  of  the  amnion  (Fig.  60)  and  posteriorly 
with  the  mesoderm  of  the  belly-stalk.  That  portion  of 
it  which  lies  within  the  body  of  the  embryo,  in  addition  to 
giving  rise  to  the  serous  lining  of  the  parietal  layer  of 
the  pleuroperitoneum,  becomes  converted  into  mesenchyme, 
which  for  a  considerable  length  of  time  is  clearly  differen- 
tiated into  two  zones,  a  more  compact  dorsal  one  which 
may  be  termed  the  somatic  layer  proper,  and  a  thinner, 
more  ventral  vascular  zone  which  is  termed  the  membrana 
reuniens  (Fig.  61).  In  the  earlier  stages  the  somatic  layer 
proper  does  not  extend  ventrally  beyond  the  line  which 


I08  THE    MESODERMIC    SOMITES. 

passes  through  the  Hmb  buds  and  it  grows  out  into  these 
buds  to  form  an  axial  core  for  them  (Fig.  6i,  Lr),  in  which 
later  the  skeleton  of  the  limb  forms.  The  remainder  of  the 
mesoderm  lining  the  sides  and  ventral  portions  of  the  body- 
wall  is  at  first  formed  from  the  membrana  reuniens,  but  as 
development  proceeds  the  somatic  layer  gradually  extends 
more  ventrally  and  displaces,  or,  more  properly  speaking, 
assimilates  into  itself,  the  membrana  reuniens  until  finally 
the  latter  has  completely  disappeared. 

It  is  to  be  noted  that  no  part  of  the  voluntary  muscula- 
ture of  the  lateral  and  ventral  walls  of  the  neck  and  trunk 
is  derived  from  the  somatic  layer;  it  is  formed  entirely 
from  the  myotomes  which  gradually  extend  ventrally  and 
finally  come  into  contact  with  their  fellows  of  the  opposite 
side  in  the  mid-ventral  line.  Whether  the  voluntary  mus- 
culature of  the  limbs  is  also  derived  from  the  myotomes  is 
at  present  doubtful.  It  has  been  very  generally  believed 
that  the  myotomes  in  their  growth  ventrally  sent  prolonga- 
tions into  the  limb  buds  which  invested  the  axial  core  of 
mesenchyme  and  eventually  gave  rise  to  the  voluntary  mus- 
cles, and  such  may  really  be  the  case,  the  relations  of  the 
various  parts  developed  from  the  gastral  mesoderm  being 
as  represented  in  the  diagrams  composing  Fig.  6i.  The 
actual  existence  of  the  prolongations  of  the  myotomes  and 
their  conversion  into  the  limb  musculature  has,  however,  not 
yet  been  observed  and  it  is  quite  possible  that  the  limb  muscu- 
lature may  be  derived  from  the  axial  core  of  somatic  meso- 
derm from  which  the  limb  skeleton  develops. 

The  appearance  of  the  mesodermic  somites  is  an  impor- 
tant phenomenon  in  the  development  of  the  embryo,  since 
it  influences  fundamentally  the  future  structure  of  the  or- 
ganism. If  each  pair  of  mesodermic  somites  be  regarded 
as  an  element  and  termed  a  metamere  or  segment,  then  it 
may  be  said  that  the  body  is  composed  of  a  series  of  meta- 


METAMERISM. 


109 


meres,  each  more  or  less  closely  resembling  its  fellows,  and 
succeeding  one  another  at  regular  intervals.  Each  somite 
differentiates,  as  has  been  stated,  into  a  sclerotome  and  a 
myotome,  and,  accordingly,  there  will  primarily  be  as  many 
vertebrae  and  muscle  segments  as  there  are  mesodermic 
somites,  or,  in  other  words,  the  axial  skeleton  and  the  vol- 
untary muscles  of  the  trunk  are  primarily  metameric.     Nor 


MO 


Fig.   61. — Diagrams    Illustrating    the    History    of    the    Gastral 

Mesoderm. 

C,  Outer  layer  of  myotome ;  Dm,  dorsal  portion  of  myotome ;  Gr, 
genital  ridge;  /,  intestine;  Lr,  limb  bud;  Mr,  membrana  reuniens; 
A'",  nervous  system;  Nc,  notochord;  Sc,  sclerotome;  So  and  Sp, 
somatic  and  splanchnic  mesoderm;  Vm,  ventral  portion  of  myo- 
tome;   Wd,  Wolffian   duct. —  (Modified  from  KoIImajiii.) 


is  this  all.  Since  each  metamere  is  a  distinct  unit,  it  must 
possess  its  own  supply  of  nutrition,  and  hence  the  primary 
arrangement  of  the  blood-vessels  is  also  metameric,  a  branch 
passing  off  on  either  side  from  the  main  longitudinal  arteries 
and  veins  to  each  metamere.  And,  further,  each  pair  of 
muscle  segments  receives  its  own  nerves,  so  that  the  arrange- 
ment of  the  nerves,  again,  is  distinctly  metameric. 


no  METAMERISM. 

This  metamerism  is  most  distinct  in  the  neck  and  trunk 
regions,  and  at  first  only  in  the  dorsal  portions  of  these 
regions,  the  ventral  portions  showing  metamerism  only- 
after  the  extension  into  them  of  the  myotomes.  But  there 
is  clear  evidence  that  the  arrangement  extends  also  into 
the  head,  and  that  this,  like  the  rest  of  the  hody,  is  to  be 
regarded  as  composed  of  metameres.  It  has  been  seen  that 
in  the  notochordal  region  of  the  head  of  lower  vertebrates 
mesodermic  somites  are  present,  while  anteriorly  in  the  pr?e- 
chordal  region  there  are  head-cavities  which  resemble  closely 
the  mesodermic  somites,  and  may,  perhaps,  be  directly  com- 
parable to  the  somites  of  the  trunk.  There  is  reason,  there- 
fore, for  believing  that  the  fundamental  arrangement  of  all 
parts  of  the  body  is  metameric,  but  though  this  arrangement 
is  clearly  defined  in  early  embryos,  it  loses  distinctness  in 
later  periods  of  development.  But  even  in  the  adult  the 
primary  metamerism  is  clearly  indicated  in  the  arrangement 
of  the  nerves  and  of  parts  of  the  axial  skeleton,  and  careful 
study  frequently  reveals  indications  of  it  in  highly  modified 
muscles  and  blood-vessels. 

In  the  head  the  development  of  the  branchial  arches  and 
clefts  produces  a  series  of  parts  presenting  many  of  the 
.peculiarities  of  metameres,  and,  indeed,  it  has  been  a  very 
general  custom  to  regard  them  as  expressions  of  the  general 
metamerism  which  prevails  throughout  the  body.  It  is  to 
be  noted,  however,  that  they  are  produced  by  the  segmenta- 
tion of  the  ventral  mesoderm,  a  structure  which  in  the  neck 
and  trunk  regions  does  not  share  in  the  general  metamerism, 
and,  furthermore,  recent  observations  on  the  cranial  nerves 
seem  to  indicate  that  these  branchiomeres  cannot  be  regarded 
as  portions  of  the  head  metameres  or  even  as  structures 
comparable  to  these.  They  represent,  more  probably,  a 
second  metamerism  superpcjsed  upon  the  more  general  one, 
or,  indeed,  possibly  more  primitive  than  it,  but  whose  rela- 


/ 


LITERATURE.  Ill 

tions  can  only  be  properly  understood  in  connection  with 
a  study  of  the  cranial  nerves  (see  p.  445). 

LITERATURE. 

In  addition  to  many  of  the  papers  cited  in  the  list  at  the  close  of 
Chapter  II,  the  following  may  be  mentioned : 
/  C.  R.  Bardeen  :   "  The  Development  of  the  Musculature  of  the  Body 

Wall  in  the  Pig,  etc.,"  Johns  Hopkins  Hosp.  Rep.,  ix,  1900. 
W.  Heape:  "The  Development  of  the  Mole  (Talpa  Europsea),"  Quar- 
terly Journ.  Microsc.  Science,  xxvii,  1887. 
F.    Keibel  :    "  Zur    Entwickelungsgeschichte    der    Chorda   bei    Saugern 

(Meerschweinchen     und     Kaninchen),"     Archiv    fUr    Anat.     und 

Physiol,  Anat.  Abth.,  1889. 
<^S.   Kaestner:    "  Ueber   die    Bildung  von   animalen   Muskelfasern   aus 

dem  Urwirbel,"  Arch,  fiir  Anat.   und  Phys.,  Anat.  Abth.,  Suppl., 

1890. 
J.  KoLLMANN :  "  Die  Rumpfsegmente  menschlicher  Embryonen  von  13 

bis  35   Urwirbeln,"   Archiv  fiir  Anat.   und  Physiol.,  Anat.   Abth., 

1891. 
J.  W.  VAN  Wijhe:  "Ueber  die  Mesodermsegmente  des  Rumpfes  und 

die    Entwicklung    des    Excretionsystems    bei    Selachiern,"    Archiv 

fiir  mikrosk.  Anat.,  xxxiii,  1889. 
«^K.  W.  Zimmermann:  "Ueber  Kopfhohlenrudimente  beim'  Menschen," 

Archiv  fiir  mikrosk.  Anat.,  liii,  1898. 


CHAPTER   V. 

THE    YOLK-STALK,    BELLY-STALK,    AND    FETAL 
MEMBRANES. 

The  conditions  to  which  the  embryos  and  larvae  of  the 
majority  of  animals  must  adapt  themselves  are  so  different 
from  those  under  which  the  adult  organisms  exist  that  in 
the  early  stages  of  development  special  organs  are  very  fre- 
quently developed  which  are  of  use  only  during  the  em- 
bryonic or  larval  period  and  are  discarded  when  more 
advanced  stages  of  development  have  been  reached.  This 
remark  applies  with  especial  force  to  the  human  embryo 
which  leads  for  a  period  of  nine  months  what  may  be  termed 
a  parasitic  existence,  drawing  its-  nutrition  from  and  yielding 
up  its  waste  products  to  the  blood  of  the  parent.  In  order 
that  this  may  be  accomplished  certain  special  organs  are 
developed  by  the  embryo,  by  means  of  which  it  forms  an 
intimate  connection  with  the  walls  of  the  uterus,  which,  on 
its  part,  becomes  greatly  modified,  the  combination  of  em- 
bryonic and  maternal  structures  producing  what  are  termed 
the  deciducB,  owing  to  their  being  discarded  at  birth  when 
the  parasitic  mode  of  life  is  given  up. 

Furthermore,  it  has  already  been  seen  that  many  pecu- 
liar modifications  of  development  in  the  human  embryo 
result  from  the  inheritance  of  structures  from  more  or  less 
remote  ancestors,  and  among  the  embryonic  adnexes  are 
found  structures  which  represent  in  a  more  or  less  modi- 
fied condition  organs  of  considerable  functional  importance 
in  lower  forms.  Such  structures  are  the  yolk-stalk  and 
vesicle,  the  amnion,  and  the  allantois,  and  for  their  proper 


YOLK-STALK    AND    FETAL    MEMBRANES. 


113 


understanding  it  will  be  well  to  consider  briefly  their  devel- 
opment in  some  lower  form,  such  as  the  chick. 

At  the  time  when  the  embryo  of  the  chick  begins  to  be 
constricted  off  from  the  surface  of  the  large  yolk-mass,  a 
fold,  consisting  of  ectoderm  and  somatic  mesoderm,  arises 
just  outside  the  embryonic  area,  which  it  completely  sur- 
rounds. As  development  proceeds  the  fold  becomes  higher 
and  its  edges  gradually  draw  nearer  together  over  the  dorsal 


Fig.    62. — Diagrams    Illustrating   the   Formation    of   the   Amnion 

AND    AlLANTOIS     IN    THE    ChICK. 

Af,  Amnion  folds;  Al,  allantois;  Am,  amniotic  cavity;  Ds,  yolk,-sac. 
—  {Gegenbaiir.) 


surface  of  the  embryo  (Fig.  62,  A),  and  finally  meet  and 
fuse  (Fig.  62,  B),  so  that  the  embryo  becomes  enclosed 
within  a  sac,  which  is  termed  the  amnion  and  is  formed  by 
the  fusion  of  the  layers  which  constituted  the  inner  wall  of 
the  fold.  The  layers  of  the  outer  wall  of  the  fold  after 
fusion  form  part  of  the  general  ectoderm  and  somatic  meso- 
derm which  make  up  the  outer  wall  of  the  ovum  and  together 


114  YOLK-STALK    AND    FETAL    MEMBRANES. 

are  known  as  the  serosa,  corresponding  to  the  chorion  of 
the  mammalian  embryo.  The  space  which  occurs  between 
the  amnion  and  the  serosa  is  a  portion  of  the  extra- 
embryonic coelom  and  is  continuous  with  the  embryonic 
pleuroperitoneal  cavity. 

In  the  ovum  of  the  chick,  as  in  that  of  the  reptile,  the 
protoplasmic  material  is  limited  to  one  pole  and  rests  upon 
the  large  yolk-mass.  As  development  proceeds  the  germ 
layers  gradually  extend  around  the  yolk-mass  (compare 
Fig.  62,  A-C)  and  eventually  completely  enclose  it,  the  yolk- 
mass  coming  to  lie  within  the  endodermal  layer,  which, 
together  with  the  splanchnic  mesoderm  which  lines  it,  forms 
what  is  termed  the  yolk-sac.  As  the  embryo  separates  from 
the  yolk-mass  the  yolk-sac  is  constricted  in  its  proximal  por- 
tion and  so  differentiated  into  a  yolk-stalk  and  a  yolk-sac, 
the  contents  of  the  latter  being  gradually  absorbed  by  the 
embryo  during  its  gTOwth,  its  walls  and  those  of  the  stalk 
being  converted  into  a  portion  of  the  embryonic  digestive 
tract. 

In  the  meantime,  however,  from  the  posterior  portion 
of  the  digestive  tract,  behind  the  point  of  attachment  of 
the  yolk-sac,  a  diverticulum  has  begun  to  form  (Fig.  62,  A). 
This  increases  in  size,  projecting  into  the  extra-embryonic 
portion  of  the  pleuroperitoneal  cavity  and  pushing  before 
it  the  splanchnic  mesoderm  which  lines  the  endoderm  (Fig. 
62,  B  and  C).  This  is  the  allantois,  which,  reaching  a  very 
considerable  size  in  the  chick  and  applying  itself  closely  to 
the  inside  of  the  serosa,  serves  as  a  respiratory  and  excretory 
organ  for  the  embryo,  for  which  purpose  its  walls  are  richly 
supplied  with  blood-vessels,  the  allantoic  arteries  and  veins. 

Toward  the  end  of  the  incubation  period  both  the  am- 
nion and  allantois  begin  to  undergo  retrogressive  changes, 
and  just  before  the  hatching  of  the  young  chick  they  become 
completely  dried  up  and  closely  adherent  to  the  egg-shell, 


THE    AMNION.  115 

at  the  same  time  separating  from  their  point  of  attachment 
to  the  body  of  the  young  chick,  so  that  when^the  chick  leaves 
the  egg-sheU  it  bursts  through  the  dried-up  membranes  and 
leaves  them  behind  as  useless  structures. 

The  Amnion. — Turning  now  to  the  human  embryo,  it 
will  be  found  that  the  same  organs  are  present,  though  some- 
what modified  either  in  the  mode  or  the  extent  of  their 
development.  A  well-developed  amnion  occurs,  arising,  how- 
ever, in  a  very  different  manner  from  what  it  does  in  the 
chick;  a  large  yolk-sac  occurs  even  though  it  contains  no 
yolk ;  and  an  allantois  which  has  no  respiratory  or  excretory 
functions  is  present,  though  in  a  somewhat  degenerated  con- 
dition. It  has  been  seen  from  the  description  of  the  earliest 
stages  of  development  that  the  processes  which  occur  in  the 
lower  forms  are  greatly  abbreviated  in  the  human  embryo. 
The  enveloping  layer,  instead  of  gradually  extending  from 
one  pole  to  enclose  the  entire  ovum,  develops  in  situ  during 
the  stages  immediately  succeeding  segmentation,  and  the 
extra-embryonic  mesoderm,  instead  of  growing  out  from 
the  embryo  to  enclose  the  yolk-sac,  splits  off  directly  from 
the  enveloping  layer.  The  earliest  stages  in  the  develop- 
ment of  the  amnion  are  not  yet  known  for  the  human  em- 
bryo, but  from  the  condition  in  which  it  is  found  in  the 
Peters  embryo  (Fig.  35)  and  in  the  embryo  z'.H.  of  von 
Spec  (Fig.  37)  it  is  probable  that  it  arises,  not  by  the  fusion 
of  the  edges  of  a  fold,  as  in  the  chick,  but  by  a  vacuolization 
of  a  portion  of  the  inner  cell-mass,  as  has  been  described 
as  occurring  in  the  bat  (p.  57).  It  is,  then,  a  closed  cavity 
from  the  very  beginning,  the  floor  of  the  cavity  being  formed 
by  the  embryonic  disk,  its  posterior  wall  by  the  anterior  sur- 
face of  the  belly-stalk,  while  its  roof  and  sides  are  thin  and 
composed  of  a  single  layer  of  flattened  ectodermal  cells  lined 
on  the  outside  by  a  layer  of  mesoderm  continuous  with  the 
somatic  mesoderm  of  the  embryo  and  the  mesoderm  of  the 
belly-stalk  (Fig.  63,  A). 


ii6 


THE    AMNION. 


When  the  bending  downward  of  the  peripheral  portions 
of  the  embryonic  disk  to  close  in  the  ventral  surface  of  the 
embryo  occurs,  the  line  of  attachment  of  the  amnion  to  the 
disk  is  also  carried  ventrally  (Fig.  63,  B),  so  that  when 
the  constriction  off  of  the  embryo  is  practically  completed, 


Fig.  62,. — Diagrams  Illustrating  the  Formation  of  the  Umbilical 

Cord. 

The  heavy  black  hne  represents  the  embryonic  ectoderm ;  the  dotted 
hne  represents  the  line  of  reflexion  of  the  body  ectoderm  into  that 
of  the  amnion.  Ac,  Amniotic  cavity;  Al,  allantois;  Be,  extra- 
embryonic coelom ;  Bs,  belly-stalk ;  Ch,  chorion ;  P,  placenta ;  Uc, 
umbilical  cord ;  V ,  chorionic  villi ;  Ys,  yolk-sac. 

the  amnion  is  attached  anteriorly  to  the  margin  of  the 
umbilicus  and  posteriorly  to  the  extremity  of  the  band  of 
ectoderm  lining  what  may  now  be  considered  the  posterior 
surface  of  the  belly-stalk,  while  at  the  sides  it  is  attached 


THE    AMNION.  11/ 

along  an  oblique  line  joining  these  two  points  (Fig.  63,  B 
and  C,  in  which  the  attachment  of  the  amnion  is  indicated 
by  the  broken  line). 

Leaving  aside  for  the  present  the  changes  which  occur 
in  the  attachment  of  the  amnion  to  the  embryo  (see  p. 
123),  it  may  be  said  that  during  the  later  growth  of  the 
embryo  the  amniotic  cavity  increases  in  size  until  finally 
its  wall  comes  into  contact  with  the  chorion,  the  extra- 
embryonic body-cavity  being  thus  practically  obliterated 
(Fig.  63,  D),  though  no  actual  fusion  of  amnion  and  chorion 
occurs.  Suspended  by  the  umbilical  cord,  which  has  by  this 
time  developed,  the  embryo  floats  freely  in  the  amniotic 
cavity,  which  is  filled  by  a  fluid,  the  liquor  amnii,  whose 
origin  is  involved  in  doubt,  some  authors  maintaining  that 
it  infiltrates  into  the  cavity  from  the  maternal  tissues,  while 
others  hold  that  a  certain  amount  of  it  at  least  is  derived 
from  the  embryo.  It  is  a  fluid  with  a  specific  gravity  of 
about  1.003  and  contains  about  i  per  cent,  of  solids,  prin- 
cipally albumin,  grape-sugar,  and  urea,  the  last  constituent 
probably  coming  from  the  embryo.  When  present  in  great- 
est quantity, — that  is  to  say,  at  about  the  beginning  of  the 
last  month  of  pregnancy, — it  varies  in  amount  between  one- 
half  and  three- fourths  of  a  liter,  but  during  the  last  month 
it  diminishes  to  about  half  that  quantity.  To  protect  the 
epidermis  of  the  fetus  from  maceration  during  its  prolonged 
immersion  in  the  liquor  amnii,  the  sebaceous  glands  of  the 
skin  at  about  the  sixth  month  of  development  pour  out  upon 
the  surface  of  the  body  a  white  fatty  secretion  known  as 
the  vernix  caseosa. 

During  parturition  the  amnion,  as  a  rule,  ruptures  as  the 
result  of  the  contraction  of  the  uterine  walls  and  the  liquor 
amnii  escape  as  the  "  waters,"  a  phenomenon  which  nor- 
mally precedes  the  delivery  of  the  child.  As  a  rule,  the  rup- 
ture is  sufficiently  extensive  to  allow  the  passage  of  the  child, 


Il8  THE    YOLK-SAC. 

the  amnion  remaining  behind  in  the  uterus,  to  be  subse- 
quently expelled  along  with  the  deciduae. 

Occasionally  it  happens,  however,  that  the  amnion  is  suffi- 
ciently strong  to  withstand  the  pressure  exerted  upon  it  by 
the  uterine  contractions  and  the  child  is  born  still  enveloped 
in  the  amnion,  which,  in  such  cases,  is  popularly  known  as 
the  "  caul,"  the  possession  of  which,  according  to  an  old  super- 
stition, marks  the  child  as  a  favorite  of  fortune. 

As  stated  above,  the  liquor  amnii  varies  considerably  in 
amount  in  different  cases,  and  occasionally  it  may  be  present 
in  excessive  quantities,  producing  a  condition  known  as 
hydramnios.  On  the  other  hand,  the  amount  may  fall  con- 
siderably below  the  normal,  in  which  case  the  amnion  may 
form  abnormal  unions  with  the  embryo,  sometimes  producing 
malformations.  Occasionally  also  bands  of  a  fibrous  char- 
acter traverse  the  amniotic  cavity  and,  tightening  upon  the 
embryo  during  its  growth,  may  produce  various  malformations, 
such  as  scars,  splitting  of  the  eyelids  or  lips,  or  even  amputa- 
tion of  a  limb. 

The  Yolk-sac. — The  development  of  the  yolk-sac  in  the 
human  em1:)ryt\  its  differentiation  into  yolk-stalk  and  yolk- 
vesicle,  and  its  enclosure  within  the  umbilical  cord  have 
already  been  described.  When  these  changes  have  been 
completed,  the  vesicle  is  a  small  pyriform  structure  lying 
between  the  amnion  and  the  chorionic  mesoderm,  some  dis- 
tance away  from  the  extremity  of  the  umbilical  cord  (Fig. 
63,  D),  and  the  stalk  is  a  long  slender  column  of  cells  ex- 
tending from  the  vesicle  through  the  umbilical  cord  to  unite 
with  the  intestinal  tract  of  the  embryo.  The  vesicle  persists 
until  birth  and  may  be  found  among  the  decidual  tissues  as 
a  small  sac  measuring  from  3  to  10  mm.  in  its  longest  diam- 
eter. The  stalk,  however,  early  undergoes  degeneration, 
the  lumen  which  it  at  first  contains  becoming  obliterated 
and  its  endoderm  also  disappearing  as  early  as  the  end  of 
the  second  month  of  development.  The  portion  of  the  stalk 
which  extends  from  the  umbilicus  to  the  intestine  usually 
shares  in  the  degeneration  and  disappears,  but  in  about  3 


THE    ALLANTOIS    AND    BELLY-STALK.  II9 

per  cent,  of  cases  it  persists,  forming  a  more  or  less  exten- 
sive diverticulum  of  the  lower  part  of  the  small  intestine, 
sometimes  only  half  an  inch  or  so  in  length  and  sometimes 
much  larger.  It  may  or  may  not  retain  connection  with 
the  abdominal  wall  at  the  umbilicus,  and  is  known  as  Meckel's 
diverticulum. 

This  embryonic  rudiment  is  of  no  little  importance,  since, 
when  present,  it  is  apt  to  undergo  invagination  into  the  lumen 
of  the  small  intestine  and  so  occlude  it.  How  frequently  this 
happens  relatively  to  the  occurrence  of  the  diverticulum  may 
be  judged  from  the  fact  that  out  of  one  hundred  cases  of  occlu- 
sion of  the  small  intestine  six  were  due  to  an  invagination  of 
the  diverticulum. 

In  the  reptiles  and  birds  the  yolk-sac  is  abundantly  sup- 
plied with  blood-vessels  by  means  of  which  the  absorption 
of  the  yolk  is  carried  on,  and  even  although  the  functional 
importance  of  the  yolk-sac  as  an  organ  of  nutrition  is  almost 
nil  in  the  human  embryo,  yet  it  still  retains  a  well-developed 
blood-supply,  the  walls  of  the  vesicle,  especially,  possessing 
a  rich  network  of  vessels.  The  future  history  of  these  ves- 
sels, which  are  known  as  the  oinphalo-mesenteric  vessels, 
will  be  described  later  on. 

The  Allantois  and  Belly-stalk. — It  has  been  seen  that  in 
reptilian  and  avian  embryos  the  allantois  reaches  a  high 
degree  of  development  and  functions  as  a  respiratory  and 
excretory  organ  by  coming  into  contact  with  what  is  com- 
parable to  the  chorion  of  the  mammalian  embryo.  In  man 
it  subserves  similar  functions,  but  is  very  much  modified 
both  in  its  mode  of  development  and  in  its  relations  to  other 
parts,  so  that  its  resemblance  to  the  avian  organ  is  some- 
what obscured.  The  differences  depend  partly  upon  the 
remarkable  abbreviation  manifested  in  the  early  develop- 
ment of  the  human  embryo  and  partly  upon  the  fact  that 
the  allantois  serves  to  place  the  embryo  in  relation  with  the 
maternal  blood,  instead  of  with  the  external  atmosphere, 


I20 


THE    ALLANTOIS    AND    BELLY-STALK. 


as  is  the  case  in  the  egg-laying  forms.  Thus,  the  endo- 
dermal  portion  of  the  allantois,  instead  of  arising  from  the 
intestine  and  pushing  before  it  a  layer  of  splanchnic  meso- 
derm to  form  a  large  sac  lying  freely  in  the  extra-embryonic 
portion  of  the  body-cavity,  appears  in  the  human  embryo 
before  the  intestine  has  differentiated  from  the  yolk-sac  and 
pushes  its  way  into  the  solid  mass  of  mesoderm  which  forms 
the  belly-stalk  (Fig.  63,  A).  To  understand  the  signifi- 
cance of  this  process  it  is  necessary  to  recall  the  abbrevia- 
tion in  the  human  embryo  of  the  development  of  the  extra- 
embryonic mesoderm  and  body-cavity.  Instead  of  grow- 
ing out  from  the  embryonic  area,  as  it  does  in  the  lower 
forms,  this  mesoderm  develops  in  siht  by  splitting  off  from 
the  layer  of  enveloping  cells  and,  furthermore,  the  extra- 
embryonic body-cavity  arises  by 
a  splitting  of  the  mesoderm  so 
formed  before  there  is  any  trace 
of  a  splitting  of  the  embryonic 
mesoderm  ( Figs.  36  and  35).  The 
belly-stalk,  whose  development 
from  a  portion  of  the  inner  cell- 
mass  has  already  been  traced 
(p.   70),  is  to  be  regarded  as  a 


<4« 

Fig.  64. — Transverse  Sec- 
tion THROUGH  THE  BeLLY-  .  .      , 

STALK  OF  AN  Embryo  OF    portiou  of  thc  body  01  the  em- 

2.15    MM. 

Aa,  Umbilical  (allantoic) 
artery;  All,  allantois;  am, 
amnion ;  Va,  umbilical 
(allantoic)     vein. —  (His.) 


br5^o,  since  the  ectoderm  which 
covers  one  surface  of  it  resem- 
bles exactly  that  of  the  embry- 
onic disk  and  shows  an  exten- 
sion backward  of  the  medullary  groove  upon  its  surface 
(Fig.  64).  The  mesoderm,  therefore,  of  the  belly-stalk  is 
to  be  regarded  as  a  portion  of  the  embryonic  mesoderm 
wdiich  has  not  yet  undergone  a  splitting  into  somatic 
and  splanchnic  layers,  and,  indeed,  it  never  does  undergo 
such  a  splitting,  so  that  there  is  no  body-cavity  into 
which  the  endodermal  allantoic  diverticulum  can  grow. 


THE    ALLANTOIS    AND    BELLY-STALK.  121 

But  this  does  not  account  for  all  the  peculiarities  of  the 
human  allantois.  In  the  birds,  and  indeed  in  the  lower 
oviparous  mammals,  the  endodermal  portion  of  the  allan- 
tois is  equally  developed  with  the  mesodermal  portion,  the 
allantois  being  an  extensive  sac  whose  cavity  is  filled  with 
fluid,  and  this  is  also  true  of  such  mammals  as  the  marsu- 
pials, the  rabbit,  and  the  ruminants.  In  man,  however,  the 
endodermal  diverticulum  never  becomes  a  sac-like  structure, 
but  is  a  slender  tube  extending  from  the  intestine  to  the 
chorion  and  lying  in  the  substance  of  the  mesoderm  of  the 
belly-stalk  (Fig.  63,  D),  the  greater  portion  of  which  is 
to  be  regarded  as  homologous  with  the  relatively  thin  layer 
of  splanchnic  mesoderm  covering  the  endodermal  diverticu- 
lum of  the  chick.  An  explanation  of  this  disparity  in  the 
development  of  the  mesodermal  and  endodermal  portions 
of  the  human  allantois  is  perhaps  to  be  found  in  the  altered 
conditions  under  which  the  respiration  and  secretion  take 
place.  In  all  forms,  the  lower  as  well  as  the  higher,  it  is 
the  mesoderm  which  is  the  more  important  constituent  of 
the  allantois,  since  in  it  the  blood-vessels,  upon  whose  pres- 
ence the  physiological  functions  depend,  arise  and  are  em- 
bedded. In  the  birds  and  oviparous  mammals  there  are  no 
means  by  which  excreted  material  can  be  passed  to  the  exte- 
rior of  the  ovum,  and  it  is,  therefore,  stored  up  within  the 
cavity  of  the  allantois,  the  allantoic  fluid. containing  consid- 
erable quantities  of  nitrogen,  indicating  the  presence  of 
urea.  In  the  higher  mammals  the  intimate  relations  which 
develop  between  the  chorion  and  the  uterine  walls  allow  of 
the  passage  of  excreted  fluids  into  the  maternal  blood ;  and 
the  more  intimate  these  relations,  the  less  necessity  there 
is  for  an  allantoic  cavity  in  which  excreted  fluid  may  be 
stored  up.  The  difference  in  the  development  of  the  cavity 
in  the  ruminants,  for  example,  and  man  depends  probably 
upon  the  greater  intimacy  of  the  union  between  ovum  and 
12 


122  THE    ALLANTOIS    AND    BELLY-STALK. 

iiteruB  in  the  latter,  the  arrangement  for  the  passage  of 
the  excreted  material  into  the  maternal  blood  being  so  per- 
fect that  there  is  practically  no  need  for  the  development 
of  an  allantoic  cavity. 

The  portion  of  the  endodermal  diverticulum  which  is 
enclosed  within  the  umbilical  cord  persists  until  birth  in  a 
more  or  less  rudimentary  condition,  but  the  intra-embryonic 
portions  of  the  allantois  reach  a  greater  development,  the 
more  proximal  portions  acquiring  a  cavity  of  considerable 
extent  and  forming  the  urogenital  sinus  and  the  urinary 
bladder,  while  the  portion  intervening  between  the  apex  of 
the  bladder  and  the  umbilicus  becomes  converted  into  a 
solid  cord  of  fibrous  tissue  termed  the  urachus. 

Occasionally  a  lumen  persists  in  the  urachal  portion  of  the 
allantois  and  may  open  to  the  exterior  at  the  umbilicus,  in 
which  case  urine  from  the  bladder  may  escape  at  the  umbilicus. 

Since  the  allantois  in  the  human  embryo,  as  well  as  in 
the  lower  forms,  is  responsible  for  respiration  and  excre- 
tion, its  blood-vessels  are  well  developed.  They  are  repre- 
sented in  the  belly-stalk  by  two  veins  and  two  arteries  (Fig. 
64),  known  in  human  embryology  as  the  mnbilical  veins 
and  arteries,  which  extend  from  the  body  of  the  embryo 
out  to  the  chorion,  there  branching  repeatedly  to  enter  the 
numerous  chorionic  villi  by  which  the  embryonic  tissues  are 
placed  in  relation  with  the  maternal. 

The  Umbilical  Cord. — During  the  process  of  closing  in 
of  the  ventral  surface  of  the  embryo  a  stage  is  reached  in 
which  the  embryonic  and  extra-embryonic  portions  of  the 
body-cavity  are  completely  separated  except  for  a  small 
area,  the  iimhilicns,  through  which  the  yolk-stalk  passes 
out  (Fig.  63,  B).  At  the  edges  of  this  area  in  front  and  at 
the  sides  the  embryonic  ectoderm  and  somatic  mesoderm 
become  continuous  with  the  corresponding  layers  of  the 
amnion,  hut  posteriorly  the  line  of  attachment  of  the  am- 


THE    UMBILICAL    CORD.  I  23 

nion  passes  up  upon  the  sides  of  the  belly-stalk  (Fig.  63, 
B),  so  that  the  whole  of  the  ventral  surface  of  the  stalk 
is  entirely  uncovered  by  ectoderm,  this  layer  being  limited 
to  its  dorsal  surface  (Fig.  64).  In  subsequent  stages  the 
embryonic  ectoderm  and  somatic  mesoderm  at  the  edges 
of  the  umbilicus  grow  out  ventrally,  carrying  with  them 
the  line  of  attachment  of  the  amnion  and  forming  a  tube 
which  encloses  the  proximal  part  of  the  yolk-stalk.  The 
ectoderm  of  the  belly-stalk  at  the  same  time  extending  more 
laterally,  the  condition  represented  in  Fig.  6t„  C,  is  pro- 
duced, and,  these  processes  continuing,  the  entire  belly- 
stalk,  together  with  the  yolk-stalk,  becomes  enclosed  within 
a  cylindrical  cord  extending  from  the  ventral  surface  of 
the  body  to  the  chorion  and  forming  the  umbilical  cord 
(Fig.  63,  D). 

From  this  mode  of  development  it  is  evident  that  the 
cord  is,  strictly  speaking,  a  portion  of  the  embryo,  its  sur- 
faces being  completely  covered  by  embryonic  ectoderm,  the 
amnion  being  carried  during  its  formation  further  and 
further  from  the  umbilicus  until  finally  it  is  attached  around 
the  distal  extremity  of  the  cord. 

In  enclosing  the  yolk-stalk  the  umbilical  cord  encloses 
also  a  small  portion  of  what  was  originally  the  extra- 
embryonic body-cavity  surrounding  the  yolk-stalk.  A  sec- 
tion of  the  cord  in  an  early  stage  of  its  development  (Fig. 
65,  A)  will  show  a  thick  mass  of  mesoderm  occupying  its 
dorsal  region;  this  represents  the  mesoderm  of  the  belly- 
stalk  and  contains  the  allantois  and  the  umbilical  arteries 
and  vein  (the  two  veins  originally  present  in  the  belly-stalk 
having  fused),  while  toward  the  ventral  surface  there  will 
be  seen  a  distinct  cavity  in  which  lies  the  yolk-stalk  with  its 
accompanying  blood-vessels.  The  portion  of  this  coelom 
nearest  the  body  of  the  embryo  becomes  much  enlarged,  and 
during  the  second  month  of  development  contains  some  coils 


124 


THE    UMBILICAL    CORD. 


-uv 


al  - 


ua 


UV 


Fig.  65. — Transverse  Sections  of  the  Umbilical  Cord  of  Embryos 

of  {a)  1.8  cm.  and  (5)  25  cm. 
al,  Allantois;    c,  coclom;    ua,  umbilical   artery;   uv,  umbilical   vein;   ys 

yolk-stalk. 


THE    UMBILICAL    CORD.  125 

of  the  small  intestine,  but  later  the  entire  cavity  becomes 
more  and  more  encroached  upon  by  the  growth  of  the  meso- 
derm, and  at  about  the  fourth  month  is  entirely  obliterated. 
A  section  of  the  cord  subsequent  to  that  period  of  develop- 
ment will  show  a  solid  mass  of  mesoderm  in  which  are  em- 
bedded the  umbilical  arteries  and  vein,  the  allantois,  and  the 
rudiments  of  the  yolk-stalk  (Fig-.  65,  B). 

When  fully  formed,  the  umbilical  cord  measures  on  the 
average  55  cm.  in  length,  though  it  varies  considerably  in 
different  cases,  and  has  a  diameter  of  about  1.5  cm.  It 
presents  the  appearance  of  being  spirally  twisted,  an  appear- 
ance largely  due,  however,  to  the  spiral  course  pursued  by 
the  umbilical  arteries,  though  the  entire  cord  may  undergo 
a  certain  amount  of  torsion  from  the  movements  of  the 
embryo  in  the  later  stages  of  development  and  may  even 
be  knotted.  The  greater  part  of  its  substance  is  formed  by 
the  mesoderm,  the  cells  of  which  become  stellate  and  form 
a  reticulum,  the  meshes  of  which  are  occupied  by  connec- 
tive-tissue fibrils  and  a  mucous  fluid  which  gives  to  the 
tissue  a  jelly-like  consistence,  whence  it  has  received  the 
name  of  Wharton's  jelly. 

The  Chorion. — To  understand  the  developmental  changes 
which  the  chorion  undergoes  it  will  be  of  advantage  to 
obtain  some  insight  into  the  manner  in  which  the  ovum 
becomes  implanted  in  the  wall  of  the  uterus.  Nothing  is 
known  as  to  how  this  implantation  is  effected  in  the  case 
of  the  human  ovum;  it  has  already  been  accomplished  in 
the  youngest  ovum  at  present  known.  But  the  process  has 
been  observed  in  other  mammals,  and  what  takes  place  in 
Spermophilus,  for  example,  may  be  supposed  to  give  a  clue 
to  what  occurs  in  the  human  ovum.  In  the  spermophile 
the  ovum  lies  free  in  the  uterine  cavity  up  to  a  stage  at 
which  the  vacuolization  of  the  central  cells  is  almost  com- 
pleted (Fig.  66,  A).     At  one  region  of  the  covering  layer 


126 


IMPLANTATION    OF    THE    OVUM. 


the  cells  become  thicker  and  later  form  a  syncytial  projec- 
tion or  knob  which  comes  into  contact  with  the  uterine 
mucosa  (Fig.  66,  B),  and  at  the  point  of  contact  the  mucosa 
cells  undergo  degeneration,  allowing  the  knob  to  come  into 
relation  with  the  deeper  tissues  of  the  uterus  (Fig.  66,  C), 
the  process  apparently  being  one  in  which  the  mucosa  cells 
are  eroded  by  the  syncytial  knob. 

It  seems  probable  that  in  the  human  ovum  the  process  is 


B 


^^?  ^ 


Fig.  66. — Successive  Stages  in  the  Implantation  of  the  Ovum  of 

THE     SpERMOPHILE. 

a,  syncytial  knob;   k,  inner  cell  mass. —  (Rcjsck.) 

at  first  of  a  similar  nature  and  that  as  the  covering  layer 
cells  come  into  contact  with  the  deeper  layers  of  the  uterus, 
these  too  are  eroded,  and,  the  uterine  blood-vessels  being 
included  in  the  erosion  process,  an  extravasation  of  blood 
plasma  and  corpuscles  occurs  in  the  vicinity  of  the  burrow- 
ing ovum.  In  the  meantime  the  ovum  has  increased  con- 
siderably in   size,   its   growth   in  these  early  stages  being 


IMPLANTATION    OF    THE    OVUM. 


127 


Fig.  67. — Diagrams  Illustrating  the  Implantation   of  the  Ovum. 

ac,  amniotic  cavity;  bs,  belly-stalk;  cf,  chorion  frondosum;  cl,  chorion 
teve;  dc,  decidua  capsularis;  ic.  inner  cell  mass;  .y,  space  surround- 
ing ovum  which  becomes  the  intervillous  space;  um,  uterine 
mucosa ;  v,  chorionic  villus ;  ys,  yolk  sac. 


128  THE    CHORIONIC    VILLI. 

especially  rapid,  and  the  area  of  contact  consequently  in- 
creases in  size,  entailing  continued  erosion  of  the  uterine 
mucosa.  At  the  same  time,  too,  the  uterine  tissues  sur- 
rounding the  ovum  grow  up  around  it,  forming  at  first  as 
it  were  a  circular  wall  (Fig.  67,  A),  and  eventually  com- 
pletely enclose  it,  forming  an  envelope  known  as  the  decidua 
capsularis  or  reflexa.  The  blood  extravasation  is  now  con- 
tained within  a  closed  space  bounded  on  the  one  hand  by 
the  uterine  tissues  and  on  the  other  by  the  wall  of  the 
ovum  (Fig.  67,  B). 

Over  either  the  whole  or  a  greater  portion  of  the  surface 
of  the  ovum  processes,  termed  chorionic  villi,  now  begin 
to  grow  out  from  the  chorion  into  the  surrounding  blood 
space  (Fig.  67,  B),  some  floating  freely  in  the  space,  while 
others  traverse  it  and  come  into  contact  by  their  extremities 
with  the  unaltered  uterine  tissues,  forming  what  are  termed 
fixation  villi.  Later  the  portion  of  the  blood  space  bounded 
by  the  decidua  capsularis  disappears  and  with  it  the  villi 
from  the  corresponding  portion  of  the  chorion,  so  that  this 
latter  structure  becomes  differentiated  into  two  regions 
(Fig.  67,  C),  one  which  is  destitute  or  practically  so  of  viUi, 
the  chorion  Iccve,  and  one,  the  chorion  frondosum,  corre- 
sponding to  the  attachment  of  the  belly-stalk,  provided  with 
them.  The  blood  space  into  which  the  villi  project  is  usu- 
ally termed  the  intervillous  space. 

The  villi  are  at  first  irregularly  lobed  processes,  formed 
by  a  solid  mass  of  ectodermal  trophoderm  cells.  As  devel- 
opment proceeds  the  lobes  become  much  more  slender  and 
branch  so  that  each  villus  assumes  a  dendritic  form.  In 
the  meantime,  however,  processes  from  the  chorionic  meso- 
derm grow  out  into  each  villus,  extending  out  even  into  the 
terminal  branches  and  forming  a  central  core  in  which 
blood-vessels  develop,  which  become  continuous  with  the 
umbilical  arteries  and  veins.     When  this  has  occurred,  the 


THE    CHORIONIC    VILLI.  1 29 

ectoderm  differentiates  into  two  layers,  a  superficial  one  in 
which  the  cellTbonndaries  disappear  so  that  it  consists  of  a 
continuous  layer  of  protoplasm  in  which  numerous  nuclei 
are  embedded  (Fig.  69,  A,  s)  and  which  is  termed  the 
syncytium,  and  an  inner  one,  consisting  of  well-defined  cells 
arranged  in  a  single  layer  and  termed  the  Langhans 
cells  (Ic). 

It  may  be  stated  that  the  exact  significance  of  these  two 
layers  is  still  under  discussion,  some  authors  believing  the 
Langhans  cells  to  be  mesodermal,  while  others,  admitting  that 


Fig.   68. — Two   Villi   from    the   Chorion   of   an    Embryo   of   7    mm. 

they  are  ectodermal,  maintain  the  view  that  the  syiic3^tium  is 
really  maternal  tissue.  The  view  here  presented  is  most  in 
accord  with  the  more  recent  observations  (Minot,  Peters,  Mar- 
chand,  Rossi  Doria). 

As  development  proceeds  the  villi,  which  are  at  first  dis- 
tributed evenly  over  the  chorion  frondosum,  are  separated 
into  groups  termed  cotyledons  (Fig.  70)  by  the  growth 
into  the  intervillous  space  of  trabeculse  from  the  walls  of 


no 


THE    CHORIONIC    VILLI. 


Fig.    6g. — Transverse    Sections    through    Chorionic    Villi    in    (A) 

THE  Fifth  and   (B)   the  Seventh  Month  of  Development. 

f/.   Canalized   fibrin;    Ic,   Langhans   cells;   s,   syncytium. —  {A   zvhich   is 

more  highly  magnified  lliaii  B,  from  Szymonozvics;  B  from  Miiiot.) 


THE    CHORIONIC    VILLI.  I3I 

the  uterus,  the  fixation  vilH  becoming  connected  with  these 
septa  as  well  as  with  the  general  uterine  wall.  The  ecto- 
derm of  the  villi  also  undergoes  certain  changes  with  ad- 
vancing growth,  the  layer  of  Langhans  cells  disappearing 
except  in  small  areas  scattered  irregularly  in  the  villi,  and 
the  syncytium,  though  persisting,  undergoes  local  thicken- 
ings which  become  replaced,  more  or  less  extensively,  by 
depositions  of  fibrin  (Fig.  6S,  B,  cf).         '  h^rl9^ 

The  changes  which  occur  during  the  later  stages  of  devel- 


FiG.   70. — Mature   Placenta   after   Separation   from  the  Uterus. 

c.  Cotyledons ;   ch,  chorion,   amnion,   and   decidua  vera ;   um,  umbilical 

cord. —  (  Kollmann. ) 

opment  in  the  chorion  are  very  similar  to  those  described 
for  the  villi.  Thus,  the  mesoderm  thickens,  its  outermost 
layers  becoming  exceedingly  fibrillar  in  structure,  while 
the  ectoderm  differentiates  into  two  layers,  the  outer  of 
which  is  syncytial  while  the  inner  is  cellular,  and  later  still, 
as  in  the  villi,  the  syncytial  layer  is  replaced  in  irregular 
patches  by  a  peculiar  form  of  fibrin  which  is  traversed  by 
flattened  anastomosing  spaces  and  to  which  Minot  has  ap- 
plied the  name  canalized  fibrin  (Fig.  71). 


132 


THE    DECIDU^. 


The  Deciduae. — In  connection  with  the  phenomenon  of 
menstruation  periochc  akerations  occur  in  the  mucous  mem- 
brane of  the  uterus.  If  during-  one  of  these  periods  a 
fertihzed  ovum  reaches  the  uterus,  the  desquamation  of 
portions  of  the  epithehum  does  not  occur  nor  is  there  any 


:v;jj^|a.-,;v- ;;:■;:  :jaf5X'^i; 


ntES 


Fig.  71. — Section  through  the  Placental  Chorion  of  an   Embryo 

OF    Seven    Months. 
c,  Cell  layer;  ep,  remnants  of  epithelium;  fb,  fibrin  layer;  mes,  meso- 
derm.—  (Minot.) 

appreciable  hemorrhage  into  the  cavity  of  the  uterus;  the 
uterine  mucosa  remains  in  what  is  practically  the  ante- 
menstrual    condition    until    the    conclusion    of    pregnancy, 


THE    DECIDU^.  133 

when,  after  the  birth  of  the  fetus,  a  considerable  portion  of 
its  thickness  is  expelled  from  the  uterus,  forming  what  is 
termed  the  deciduce.  In  other  words,  the  sloughing-  of  the 
uterine  tissue  which  concludes  the  process  of  menstruation 
is  postponed  until  the  close  of  pregnancy,  and  then  takes 
place  simultaneously  over  the  whole  extent  of  the  uterus. 
Of  course,  the  changes  in  the  uterine  tissues  are  somewhat 


Fig.  '72. — Diagram  showing  the  Relations  of  the  Fetal  Membranes. 

Am,  Amnion;  Ch,  chorion;   M,  muscular  wall  of  uterus;   C,  decidua 

capsularis ;  B,  decidua  basalis ;   V,  decidua  vera ;   Y,  yolk-stalk. 


more  extensive  during  pregnancy  than  during  menstrua- 
tion, but  there  is  an  undoubted  fundamental  similarity  in 
the  changes  during  the  two  processes. 

The  human  ovum  comes  into  direct  apposition  with  only 
a  small  portion  of  the  uterine  wall,  and  the  changes  which 


134 


THE    DECIDU^. 


this  portion  of  the  wall  undergoes  differ  somewhat  from 
those  occurring  elsewhere.  Consequently  it  becomes  pos- 
sible to  divide  the  deciduse  into  ( i )  a  portion  which  is  not 
in  direct  contact  with  the  ovum,  the  dccidiia  vera  (Fig. 
yz,  V)  and  (2)  a  portion  which  is.  The  latter  portion  is 
again  capable  of  division.  The  ovum  becomes  completely 
embycdded  in  the  mucosa,  but,  as  has  been  pointed  out,  the 


Pig   yi^, — Surface  View  of  Half  of  the  Decidua  Vera  at  the  End 

OF  the  Third  Week  of  Gestation. 
d,  Mucous  membrane  of  the  Fallopian  tubes ;   ds,  prolongation  of  the 

vera  toward  the  cervix  uteri;  pp,  papillae;  rf,  marginal  furrow. — 

{Kolhnann.) 

chorionic  villi  reach  their  full  development  only  over  that 
portion  of  the  chorion  to  which  the  belly-stalk  is  attached. 
The  decidua  which  is  in  relation  to  this  chorion  frondosum 
undergoes  much  more  extensive  modifications  than  that  in 
relation  to  the  chorion  la;ve,  and  to  it  the  name  of  decidua 
hasalis  {decidua  serotina)    (Fig.  72,  B)   is  applied,  while 


THE    DECIDUA    VERA. 


135 


the  rest  of  the  decidua  which  encloses  the  ovum  is  termed 
the  decidua  capsidaris  (decidua  reiiexa)   (C). 

The  changes  which  give  rise  to  the  decidua  vera  may 
first  be  described  and  those  occurring  in  the  others  consid- 
ered in  succession. 

{a)  Decidua  vera. — On  opening  a 
uterus  during  the  fourth  or  fifth  month 
of  pregnancy,  when  the  decidua  vera 
is  at  the  height  of  its  development, 
the  surface  of  the  mucosa  presents  a 
corrugated  appearance  and  is  tra- 
versed by  irregular  and  rather  deep 
grooves  (Fig.  73).  This  appearance 
ceases  at  the  internal  orifice,  the  mu- 
cous membrane  of  the  cervix  uteri 
not  forming  a  decidua,  and  the  de- 
cidu£e  of  the  two  surfaces  of  the 
uterus  are  separated  by  a  distinct 
furrow  known  as  the  marginal  groove. 


1# 


Fig.   74. — Diagrammatic   Sections   of  the  Uterine   Mucosa,   A,  in 
THE     Non-pregnant     Uterus,   and    5,    at    the    Beginning    of 
Pregnancy. 
c.    Stratum    compactum ;    gl,   the    deepest    portions    of   the    glands ;    m, 
muscular  layer;  spj  stratum  spongiosum. —  (Kttiidrat  and  Eiigeh)iaini.} 

In  sections  the  mucosa  is  found  to  have  become  greatly, 
thickened,  frequently  measuring^  i  cm.  in  thickness,  and 
its  glands  have  undergone  very  considerable  modification. 
Normally  almost  straight    (Fig.   74,  A),  they  increase  in 


136  THE    DECIDUA    VERA. 

length,  not  only  keeping  pace  with  the  thickening  of  the 
mucosa,  but  surpassing  its  growth,  so  that  they  become  very 
much  contorted  and  are,  in  addition,  considerably  dilated 
(Fig.  74,  B).  Near  their  mouths  they  are  dilated,  but  not 
very  much  contorted,  while  lower  down  the  reverse  is  the 
case,  and  it  is  possible  to  recognize  three  layers  in  the  de- 
cidua,  ( I )  a  stratum  compactwn  nearest  the  lumen  of  the 
uterus,  containing  the  straight  but  dilated  portions  of  the 
glands;  (2)  a  stratum  spongiosum,  so  called  from  the  ap- 
pearance which  it  presents  in  sections  owing  to  the  dilated 
and  contorted  portions  of  the  glands  being  cut  in  various 
planes;  and  (3)  next  the  muscular  coat  of  the  uterus  a  layer 
containing  the  contorted  but  not  dilated  extremities  of  the 
glands  is  found.  Only  in  the  last  layer  does  the  epithelium 
of  the  glands  retain  its  normal  columnar  form;  elsewhere 
the  cells,  separated  from  the  walls  of  the  glands,  become 
enlarged  and  irregular  in  shape  and  eventually  degenerate. 

In  addition  to  these  changes,  the  epithelium  of  the  mucosa 
disappears  completely  during  the  first  month  of  pregnancy, 
and  the  tissue  between  the  glands  in  the  stratum  .compactum 
becomes  packed  with  large,  often  multinucleated  cells, 
which  are  termed  the  decidual  cells  and  are  probably  derived 
from  the  connective  tissue  cells  of  the  mucosa. 

iVfter  the  end  of  the  fifth  month  the  increasing  size  of 
the  embryo  and  its  membranes  exerts  a  certain  amount  of 
pressure  on  the  decidua,  and  it  begins  to  diminish  in  thick- 
ness. The  portions  of  the  glands  which  lie  in  the  stratum 
compactum  become  more  and  more  compressed  and  finally 
disappear,  while  in  the  spongiosum  the  spaces  become  much 
flattened  and  the  vascularity  of  the  whole  decidua,  at  first 
so  pronounced,  diminishes  greatly. 

{h)  Decidua  capsularis. — The  decidua  capsularis  has  also 
been  termed  the  decidua  reflexa,  on  the  supposition  that  it 
was  formed  as  a  fold  of  the  uterine  mucosa  reflected  over 


-    THE   DECIDUA    CAPSULARIS.  137 

the  ovum  after  this  had  attached  itself  to  the  uterine  wall. 
Since,  however,  the  attachment  of  the  ovum  is  to  be  re- 
garded as  a  process  of  burrowing  into  the  uterine  tissues 
(see  p.  126),  the  necessity  for  an  upgrowth  of  a  fold  is 
limited  to  an  elevation  of  the  uterine  tissues  in  the  neigh- 
borhood of  the  ovum  to  keep  pace  with  its  increasing  size. 
In  the  Peters'  ovum  (Fig.  75),  which  measured  i  mm.  m 


Sch. 


E.U. 


f 
J 


n 


i-%: 


Fig.  75. — Section  of  an  Ovum  of  i  mm.  A  Section  of  the  Embryo 
Lies  in  the  Lower  Part  of  the   Cavity  of  the  Ovum. 

D,  Decidua;  E.U.,  uterine  epithelium;  Sch,  blood-clot  closing  the  aper- 
ture left  by  the  sinking  of  the  ovum  into  the  uterine  mucosa. — 
{From  Strahl,  after  Peters.) 

diameter,  the  capsularis  was  not  c{uite  complete,  a  small 
area  at  one  pole  of  the  ovum  being  yet  unenclosed  by  it 
and  covered  only  by  a  patch  of  coagulated  blood.  But  in 
a  somewhat  older  ovum  described  by  Rossi  Doria,  whose 
cavity  measured  6X5  "^i^"*-  i"  diameter,  the  capsularis 
13 


138  THE    DECIDUA    BASALIS. 

formed  a  complete  investment.  Since  it  is  part  of  the  area 
of  contact  with  the  ovum  it  possesses  no  epithehum  upon 
the  surface  turned  toward  the  ovum,  although  in  the  earlier 
stages  its  surface  is  covered  by  an  epithelium  continuous 
with  that  of  the  decidua  vera,  and  between  it  and  the  chorion 
there  is  a  portion  of  the  blood  extravasation  in  which  the 
villi  formed  from  the  chorion  Iseve  float.  Glands  and  blood- 
vessels also  occur  in  its  walls  in  the  earlier  stag'es  of 
development. 

As  the  ovum  continues  to  increase  in  size  the  capsularis 
begins  to  show  signs  of  degeneration,  these  appearing  first 
over  the  pole  of  the  ovum  opposite  the  point  of  fixation. 
Here,  even  in  the  case  described  by  Rossi  Doria,  it  has 
become  reduced  to  a  thin  membrane  destitute  of  either 
blood-vessels  or  glands,  and  the  degeneration  gradually 
extends  throughout  the  entire  capsule,  the  blood  space  which 
it  encloses  also  disappearing.  At  about  the  fifth  month  the 
growth  of  the  ovum  has  brought  the  capsularis  in  contact 
throughout  its  whole  extent  with  the  vera,  and  it  then  ap- 
pears as  a  whitish  transparent  membrane  with  no  trace  of 
either  glands  or  blood-vessels,  and  very  possibly  it  eventu- 
ally degenerates  completely  and  disappears   (Minot). 

(c)  Decidua  basalis. — The  structure  of  the  decidua  ba- 
salis,  also  known  as  the  decidua  serotina,  is  practically  the 
same  as  that  of  the  vera  up  to  about  the  fifth  month.  It 
differs  only  in  that,  being  part  of  the  area  of  contact  of  the 
ovum,  it  loses  its  epithelium  much  earlier  and  is  also  the 
seat  of  extensive  blood  extravasations,  due  to  the  erosion 
of  its  vessels  by  the  chorionic  trophoderm.  Its  glands, 
however,  undergo  the  same  changes  as  those  of  the  vera, 
so  that  in  it  also  a  compactum  and  a  spongiosum  may  be 
recognized.  Beyond  the  fifth  month,  however,  there  is  a 
great  difference  between  it  and  the  vera,  in  that,  being  con- 
cerned with  the  nutrition  of  the  embryo,  it  does  not  partake 


THE    PLACENTA.  I  39 

of  the  degeneration  noticeable  in  the  other  decidtise,  but 
persists  until  birth,  forming  a  part  of  the  structure  termed 
the  placenta. 

The  Placenta. — This  organ,  which  forms  the  connection 
between  the  embryo  and  the  maternal  tissues,  is  composed 
of  two  parts,  separated  by  the  intervillous  space.  One  of 
these  parts  is  of  embryonic  origin,  being  the  chorion  fron- 
dosum,  while  the  other  belongs  to  the  maternal  tissues  and 
is  the  decidua  basalis.  Hence  the  terms  placenta  fetalis 
and  placenta  uterina  frequently  applied  to  the  two  parts. 
The  fully  formed  placenta  is  a  more  or  less  discoidal  struc- 
ture, convex  on  the  surface  next  the  uterine  muscularis  and 
concave  on  that  turned  toward  the  embryo,  the  umbilical 
cord  being  continuous  with  it  near  the  center  of  the  latter 
surface.  It  averages  about  3.5  cm.  in  thickness,  thinning 
out  somewhat  toward  the  edges,  and  has  a  diameter  of  15 
to  20  cm.,  and  a  weight  varying  between  500  and  1250 
grams.  It  is  situated  on  one  of  the  surfaces  of  the  uterus, 
the  posterior  more  frequently  than  the  anterior,  and  usually 
much  nearer  the  fundus  than  the  internal  orifice.  It  devel- 
ops, in  fact,  wherever  the  ovum  happens  to  become  attached 
to  the  uterine  walls,  and  occasionally  this  attachment  is  not 
accomplished  until  the  ovum  has  descended  nearly  to  the 
internal  orifice,  in  which  case  the  placenta  may  completely 
close  this  opening  and  form  what  is  termed  a  placenta 
prcevia. 

If  a  section  of  a  placenta  in  a  somewhat  advanced  stage 
of  development  be  made,  the  following  structures  may  be 
distinguished :  On  the  inner  surface  there  will  be  a  delicate 
layer  representing  the  amnion  (Fig.  76,  Am),  and  next  to 
this  a  somewhat  thicker  one  which  is  the  chorion  (Cho), 
in  which  the  degenerative  changes  already  mentioned  may 
be  observed.  Succeeding  this  comes  a  much  broader  area 
composed  of  the  large  intervillous  blood  space  in  which  lie 


THE    PLACENTA.  J4I 

sections  of  the  villi  (z'i)  cut  in  various  directions.  Then 
follows  the  stratum  compactum  of  the  basalis,  next  the 
stratum  spongiosum,  next  the  outermost  layer  of  the  mucosa 
(D"),  in  which  the  uterine  glands  retain  their  epithelium, 
and,  finally,  the  muscularis  uteri  (Ale). 

These  various  structures  which  enter  into  the  composi- 
tion of  the  placenta  have,  for  the  most  part,  been  already 
described,  and  it  remains  here  only  to  say  a  few  words  con- 
cerning the  special  structure  of  the  basal  compactum  and 
concerning  the  origin  of  the  intervillous  space  and  its  rela- 
tions to  the  villi  and  the  maternal  vessels. 

From  the-  surface  of  the  compactum  processes  arise^ 
termed  septa,  which  project  into  the  intervillous  space, 
grouping  the  villi  into  cotyledons  and  giving  attachment  to 
some  of  the  fixation  villi  (Fig.  76).  Throughout  the 
greater  extent  of  the  placenta  the  septa  do  not  reach  the 
surface  of  the  chorion,  but  at  the  periphery,  throughout  a 
narrow  zone,  they  do  come  into  contact  with  the  chorion 
and  unite  beneath  it  to  form  a  membrane  which  has  been 
termed  the  elosing  plate.  Beneath  this  lies  the  peripheral 
portion  of  the  intervillous  space,  which,  owing  to  the  ar- 
rangement of  the  septa  in  this  region,  appears  to  be  imper- 
fectly separated  from  the  rest  of  the  space  and  forms  what 
is  termed  the  marginal  sinus  (Fig.  77). 

The  probable  origin  of  the  intervillous  space  by  the  effu- 
sion of  blood  from  the  maternal  vessels  throughout  the 
area  of  contact  with  the  ovum  has  already  been  described, 
and  if  this  be  the  true  method  of  its  development,  then  it  is 
evident  that  the  fetal  villi  are  in  direct  contact  with  the 
maternal  blood  contained  in  the  space.     The  uterine  vessels 

Fig.  76. — Section  through  a  Placenta  of  Seven  Months'  Develop- 
ment. 

Am,  Amnion;  cito,  chorion;  D,  lajxr  of  decidua  containing  the  uterine 
glands;  Mc,  muscular  coat  of  the  uterus;  Ve,  maternal  blood- 
vessel;  Vi,  stalk  of  a  villus;  -in,  villi  in  section. —  (Miiiot.) 


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THE    PLACENTA.  .  143 

become  very  much  enlarged  during  pregnancy  and  those  of 
the  basahs  communicate  freely  with  the  intervillous  space, 
so  that  a  free  circulation  of  the  maternal  blood  through  the 
space  occurs.  The  villi  being  completely  immersed  in  this 
constantly  renewed  blood,  an  osmotic  interchange  takes 
place  between  the  maternal  blood  of  the  space  and  the  fetal 
blood  contained  in  the  vessels  of  the  villi,  the  maternal  blood 
transmitting  the  nutritive  materials  necessary  for  the  growth 
of  the  embryo  and  receiving  the  waste  products  of  the  fetal 
metabolism.  And  it  is  only  in  this  manner  that  the  nutri- 
tion of  the  embryo  can  take  place,  since  nowhere  is  there 
a  direct  communication  of  the  two  vascular  systems. 

It  has  been  maintained  by  many  authors  that  the  inter- 
villous space  is  lined  throughout  by  a  layer  of  cells  continuous 
with  the  endothelium  of  the  maternal  vessels,  so  that  the  fetal 
blood  is  separated  from  the  maternal,  not  only  by  the  fetal 
tissues  of  the  villi,  but  also  by  a  layer  of  maternal  tissue  (com- 
pare what  is  said  in  the  small  print  on  page  129  concerning 
the  homologies  of  the  ectodermal  layers  of  the  villi).  The 
presence  of  such  a  layer  is  certainly  what  might  be  expected, 
since,  as  Oscar  Hertwig  has  well  expressed  it,  "  the  employ- 
ment of  spaces  lying  outside  the  blood-courses  as  component 
parts  of  the  vascular  system  would  be  a  phenomenon  without 
analogy."  It  is  to  be  noted  that  the  arteries  and  veins  of  the 
decidua  basalis  do  not  communicate  by  means  of  capillaries, 
but  by  the  intervillous  space,  and  this  has  given  rise  to  the 
theory  that  the  space  is  to  be  regarded  as  an  enormously  en- 
larged capillary,  in  which  case  it  should  be  lined  throughout 
by  maternal  endothelium.  Recent  observations  on  the  lower 
mammals,  especially  the  rodents  (rabbits,  guinea-pig,  etc.), 
seem  to  show,  however,  that  the  space  owes  its  origin  to  a  true 
effusion  of  maternal  blood,  and  the  evidence  furnished  by 
Peters,  van  Heukelom  and  Rossi  Doria  from  the  study  of  its 
formation  in  very  early  human  embryos  indicates  its  origin 
in  the  human  placenta  in  the  manner  described  above. 

But  although  it  seems  certain  that  the  maternal  blood  con- 
tained in  the  intervillous  space  is  not  separated  by  maternal 
epithelium  from  the  villi,  nevertheless  it  is  probable  that  in 
later  stages  the  space  is  enclosed  by  epithelium.  On  the  fetal 
side  it  is,  of  course,  lined  by  the  chorionic  ectoderm  and  on 


144  SEPARATION    OF    THE    DECIDU^. 

the  maternal  side  either  by  endotheHum  which  has  extended 
out  over  the  maternal  tissues  from  the  vessels  which  commu- 
nicate with  the  space  or  partly  thus  and  partly  by  the  spread- 
ing out  of  the  syncytium  of  the  fixation  villi  over  the  maternal 
surface   (Rossi  Doria). 

The  erosion  of  the  maternal  tissues  by  the  chorionic  syncy- 
tium, both  during  the  implantation  of  the  ovum  and  the  forma- 
tion of  the  placenta,  is  a  most  striking  phenomenon  and  can 
hardly  fail  to  suggest  a  comparison  of  the  ovum  to  a  parasite 
sending  its  destructive  rootlets  or  haustoria  down  into  the  tis- 
sues of  its  host,  thereby  securing  for  itself  additional  possi- 
bilities for  nutrition.  Indeed,  this  idea  has  led  to  a  suggestion 
by  Rossi  Doria  that  the  formation  of  the  placenta  is  a  struggle 
between  the  parasite  and  the  maternal  tissues,  the  decidual  cells 
of  the  latter,  massing  beneath  the  intervillous  space  to  form 
what  has  been  termed  the  basal  plate,  constituting  a  line  of 
resistance  to  the  continued  encroachments  of  the  syncytium. 

The  Separation  of  the  Deciduae  at  Birth. — At  parturi- 
tion, after  the  rupture  of  the  amnion  and  the  expulsion  of 
the  fetus,  there  still  remains  in  the  uterine  cavity  the  decidune 
and  the  amnion,  which  is  in  contact  but  not  fused  with  the 
deciduae.  A  continuance  of  the  uterine  contractions,  pro- 
ducing what  are  termed  the  "  after-pains,"  results  in  the 
separation  of  the  placenta  from  the  uterine  walls,  the  separa- 
tion taking  place  in  the  deep  layers  of  the  spongiosum,  so 
that  the  portion  of  the  mucosum  which  contains  the  unde- 
generated  glands  remains  behind.  As  soon  as  the  placenta 
has  separated,  the  separation  of  the  decidua  vera  takes  place 
gradually  though  rapidly,  the  line  of  separation  again  being 
in  the  deeper  layers  of  the  stratum  spongiosum,  and  the 
whole  of  the  deciduae,  together  with  the  amnion,  is  expelled 
from  the  uterus,  forming  what  is  known  as  the  "  after- 
birth." 

Hemorrhage  from  the  uterine  vessels  during  and  after 
the  separation  of  the  deciduae  is  prevented  by  the  contrac- 
tions of  the  uterine  walls,  assisted,  according  to  some  au- 
thors, by  a  preliminary  blocking  of  the  mouths  of  the  uter- 
ine vessels  by  certain  large  polynuclear  decidual  cells  found 
during  the  later  months  of  pregnancy  in  the  outer  layers 


LITERATURE.  1 45 

of  the  decidna  basalis.  The  regeneration  of  the  uterine 
mucosa  after  parturition  has  its  starting-point  from  the 
epithehum  of  the  undegenerated  glands  which  persist,  this 
epithehum  rapidly  evolving  a  complete  mucosa  over  the 
entire  surface  of  the  uterus. 

LITERATURE. 

S.    VAN    Heukelom  :    "  Ueber    die    menschliche    Placentation,"    Archiv 

fiir  Anat.  iind  Physiol,  Anai.  Abth.,  i8g8. 
W.  His  :  "  Die  Umschliessimg  der  menschlichen  Frucht  wahrend  der 

friihesten    Zeit    des     Schwangerschafts,"    Archiv    fiir    Anat.     und 

Physiol,  Anat.  Abth.,  1897. 

F.  Keibel:   "  Zur  Entwickelungsgeschichte    der    Placenta,"   Anat.   An- 

zeiger,  iw,  1889. 
J.   KoLLMANN :   "  Die  menschlichen  Eier  von  6  mm.  Grosse,"  Archiv 
fiir  Anat.  und  Physiol,  Anat.  Abth.,  1879. 

G.  Leopold:   "Ueber  ein  sehr  Junges  menschliches  Ei   in  situ,"  Arh. 

aus  der  k'dnigl  Frauenklinik  in  Dresden,  iv,  1906. 

F.  Marchand:  "  Beobachtungen  an  jungen  menschlichen  Eiern,"  Anat. 

Hcfte,  XXI,  1903. 
J.   Merttens  :    "  Beitrage   zur   normalen   und  pathologischen   Anatomic 

der    menschlichen    Placenta,"    Zcitschrift    fiir    Geburtshulfe    und 

Gynaekol,  xxx  and  xxxi,  1894. 
C.   S.  Minot:   "Uterus  and   Embryo,"  Journal  of  Morphol,  u,   1889. 

G.  Paladino  :  "  Sur  la  genese  des  espaces  intervilleux  du  placenta 
humain  et  de  leur  premier  contenu,  comparativement  a  la  meme 
partie  chez  quelques  mammiferes,"  Archives  Ital  de  Biolog.,  xxxi 
and  xxxii,   1899. 

H.  Peters  :  "  Ueber  die  Einbettung  des  menschlichen  Eies  imd  das 
friiheste  bisher  bekannte  menschliche  Placentationsstadium,"  Leip- 
zig und  Wien,  1899. 

J.  Rejsek:  "Anheftung  (Implantation)  des  Sangetiereies  an  die 
Uteruswand,  insbesondere  des  Eies  von  Spermophilus  citellus," 
Arch,  fiir  mikrosk.  Anat.,  lxiii,   1904. 

T.  Rossi  Doria:  "Ueber  die  Einbettung  des  menschlichen  Eies,  studirt 
an  einem  kleinen  Eie  der  Zweiten  Woche,"  Arch,  fiir  Gynaek., 
Lxxvi,  1905. 

C.  Ruge  :  "  Ueber  die  menschliche  Placentation,"  Zcitschrift  fiir  Ge- 
burtshiilfe  und  Gynaekol,  xxxix,  1898. 

F.  Graf  Spee  :  "  Ueber  die  menschliche  Eikammer  und  Decidua  re- 
flexa,"   Vcrhandl  des  Anat.  Gesellsch.,  xii,  1898. 

J.  C.  Webster  :  "  Human  Placentation,"  Chicago,  1901. 

14 


PART    II. 

ORGANOGENY. 


CHAPTER    VL 

THE  DEVELOPMENT  OF  THE  INTEGUMENTARY 

SYSTEM. 

The  Development  of  the  Skin. — The  skin  is  composed 
of  two  embryologically  distinct  portions,  the  outer  epidermal 
layer  being  developed  from  the  ectoderm,  while  the  dermal 
layer  is  mesenchymatous  in  its  origin. 

The  ectoderm  covering  the  general  surface  of  the  body 
is,  in  the  earliest  stages  of  development,  a  single  layer  of 
cells,  but  at  the  end  of  the  first  month  it  is  composed  of  two 
layers,  an  outer  one,  the  epitrichium,  consisting  of  slightly 
flattened  cells,  and  a  lower  one  whose  cells  are  larger  and 
which  will  give  rise  to  the  epidermis  (Fig.  78,  A).  During 
the  second  month  the  differences  between  the  two  layers 
become  more  pronounced,  the  epitrichial  cells  assuming  a 
characteristic  domed  form  and  becoming  vesicular  in  struc- 
ture (Fig.  78,  B).  These  cells  persist  until  about  the  sixth 
month  of  development,  but  after  that  they  are  cast  off,  and, 
becoming  mixed  with  the  secretion  of  sebaceous  glands 
which  have  appeared  by  this  time,  form  a  constituent  of  the 
vernix  caseosa. 

In  the  meantime  changes  have  been  taking  place  in  the 
epidermal  layer  which  result  in  its  becoming  several  layers 
thick  (Fig.  78,  B),  the  innermost  layer  being  composed  of 
cells  rich  in  protoplasm  while  those  of  the  outer  layers  are 

147 


148 


DEVELOPMENT    OF    THE    SKIN. 


irregular  in  shape  and  have  clearer  contents.  As  develop- 
ment proceeds  the  number  of  layers  increases  and  the  super- 
ficial ones,  undergoing  a  horny  degeneration,  give  rise  to  the 
stratum  corneum,  while  the  deeper  ones  become  the  stratum 
Malpighii.  At  about  the  fourth  month  ridges  develop  on 
the  under  surface  of  the  epidermis,  projecting  downward 
into  the  dermis  (Fig.  84),  and  later  secondary  ridges  appear 
in  the  intervals  between  the  primary  ones,  while  on  the 


Fig.  78. — A,  Section  of  Skin  from  the  Dorsum  of  Finger  of  an 
Embryo  of  4.5  cm.;  B,  from  the  Plantar  Surface  of  the  Foot 
of  an  Embryo  of  10.2  cm. 

et,  Epitrichium ;   cp,  epidermis. 


palms  and  soles  ridges  appear  upon  the  outer  surface  of  the 
epidermis,  corresponding  in  position  to  the  primary  ridges 
of  the  under  surface. 

The  mesenchyme  which  gives  rise  to  the  dermis  grows 
in  from  all  sides  between  the  epidermis  and  the  outer  layer 
of  the  myotomes,  which  are  at  first  in  contact,  and  forms  a 
continuous  layer  underlying  the  epidermis  and  showing  no 
indications  of  a  segmental  arrangement.     It  becomes  con- 


DEVELOPMENT    OF    THE    SKIN. 


149 


/     // 


'^^  rA 


Tf 


^ 


Ts 


re 


T7 


r,o 


7// 


Lr 


Ls 


Si 


verted  principally  into  fibrous  con- 
nective tissue,  the  outer  layers  of 
which  are  relatively  compact,  while 
the  deeper  ones  are  looser,  forming 
the  subcutaneous  areolar  tissue.  Some 
of  the  mesenchymal  cells,  however, 
become  converted  into  non-striated 
muscle-fibers,  which  for  the  most  part 
are  few  in  number  and  associated 
with  the  hair  follicles,  though  in  cer- 
tain regions,  such  as  the  skin  of  the 
scrotum,  they  are  very  numerous  and 
form  a  distinct  layer  known  as  the 
dartos.  Some  cells  also  arrange  them- 
selves in  groups  and  undergo  a  fatty 
degeneration,  well-defined  masses  of 
adipose  tissue  embedded  in  the  lower 
layers  of  the  dermis  being  thus 
formed  at  about  the  sixth  month. 

Although  the  dermal  mesenchyme  is 
unsegmental  in  character,  yet  the  nerves 
which  send  branches  to  it  are  segmental, 
and  it  might  be  expected  that  indica- 
tions of  this  condition  would  be  retained 
by  the  cutaneous  nerves  even  in  the 
adult.  A  study  of  the  cutaneous  nerve- 
supply  in  the  adult  realizes  to  a  very 
considerable  extent  this  expectation,  the 
areas  supplied  by  the  various  nerves 
forming  more  or  less  distinct  zones, 
and  being  therefore  segmental  (Fig. 
79).  But  a  considerable  commingling 
of  adjacent  areas  has  also  occurred. 
Thus,     while    the     distribution     of    the 

Fig.  79. — Diagram  showing  the  Cutaneous 
Distribution  of  the  Spinal  Nerves. — 
{Head.') 


ISO 


DEVELOPMENT    OF    THE    SKIN. 


cutaneous  branches  of  the  fourth  thoracic  nerve,  as  determined 
experimentally  in  the  monkey  (Macacus),  is  distinctly  zonal 
or  segmental,  the  nipple  lying  practically  in  the  middle  line 
of  the  zone,  the  upper  half  of  its  area  is  also  supplied  or  over- 
lapped by  fibers  of  the  third  nerve  and  the  lower  half  by  fibers 
of  the  fifth  (Fig".  80),  so  that  any  area  of  skin  in  the  zone  is 
innervated  b}'  fibers  coming  from  at  least  two  segmental  nerves 
(Sherrington).  And,  furthermore,  the  distribution  of  each 
nerve  crosses  the  mid-ventral  line  of  the  body,  forming  a  more 
or  less  extensive  crossed  overlap. 

And  not  only  is  there  a  confusion  of  adjacent  areas  but  an 
area  may  shift  its  position  relatively  to  the  deeper  structures 


Fig.  So. — Diagram  showing  the  Overlap  of  the  ///,  IV,  and  V  In- 
tercostal Nerves  of  a  Monkey. —  (Sherrington.) 


supplied  by  the  same  nerve,  so  that  the  skin  over  a  certain 
muscle  is  not  necessarily  supplied  by  fibers  from  the  nerve 
which  supplies  the  muscle.  Thus,  in  the  lower  half  of  the 
abdomen,  the  skin  at  any  point  will  be  supplied  by  fibers  from 
higher  nerves  than  those  supplying  the  underlying  muscles 
(Sherrington),  and  the  skin  of  the  limbs  may  receive  twigs 
from  nerves  which  are  not  represented  at  all  in  the  muscle- 
supply  (second  and  third  thoracic  and  third  sacral). 

The  Development  of  the  Nails. — The  earliest  indica- 
tions of-the  development  of  the  nails  have  been  described  by 
Zander  in  embryos  of  about  nine  weeks  as  slight  thicken- 
ings of  the  epidermis  of  the  tips  of  the  digits,  these  thick- 
enings being  separated  from  the  neighboring  tissue  by  a 
faint  gToove.  Later  the  nail  areas  migrate  to  the  dorsal 
surfaces  of  the  terminal  phalanges  (Fig.  81)  and  the  grooves 


DEVELOPMENT    OF    THE    NAILS. 


151 


surrotinding  the  areas  deepen,  especially  at  their  proximal 
edges,  where  they  form  the  nail-folds  (nf),  while  distally 
thickenings  of  the  epidermis  occur  to  form  what  have  been 
termed  sole-plates  (sp),  structures  cjuite  rudimentary  in 
man,  but  largely  developed  in  the  lower  animals,  in  which 
they  form  a  considerable  portion  of  the  claws. 

The  actual  nail  substance  does  not  form,  however,  until 


•  irViZEIliZ2*.^^,/?7^ 


Fig.   81. — Longitudinal   Section   through   the   Terminal   Joint   of 

THE  Index-finger  of  an  Embryo  of  4.5  cm. 
e.  Epidermis;  ep,  epitrichium;  nf,  nail  fold;  Ph,  terminal  phalanx;  sp, 

sole  plate. 


the  embryo  has  reached  a  length  of  about  17  cm.  By  this 
time  the  epidermis  has  become  several  layers  thick  and  its 
outer  layers,  over  the  nail  areas  as  well  as  elsewhere,  have 
bcome  transformed  into  the  stratum  corneum  (Fig.  82,  sc), 
and  it  is  in  the  deep  layers  of  this  (the  stratum  lucidum) 
that  keratin  granules  develop  in  cells  which  degenerate  to 
give  rise  to  the  nail  substance  (w).  At  its  first  formation, 
accordingly,  the  nail  is  covered  by  the  outer  layers  of  the 
stratum  corneum  as  well  as  by  the  epitrichium,  the  two 


152 


DEVELOPMENT    OF    THE    NAILS. 


sp- 


sc 


ep 


nb 


together  forming  what  has  been  termed  the  eponychium 
(Fig.  82,  cp).  The  epitrichium  soon  disappears,  however, 
leaving  only  the  outer  layers  of  the 
stratum  corneum  as  a  covering,  and 
this  also  later  disappears  with  the 
exception  of  a  narrow  band  sur- 
rounding the  base  of  the  nail  which 
persists  as  the  perionyx. 

The  formation  of  the  nail  begins 
in  the  more  proximal  portion  of  the 
nail  area  and  its  further  growth  is 
by  the  addition  of  new  keratinized 
cells  to  its  proximal  edge  and  lower 
surface,  these  cells  being  formed 
only  in  the  proximal  part  of  the  nail 
bed  in  a  region  marked  by  its  whit- 
ish color  and  termed  the  lunula. 

The  first  appearance  of  the  nail-areas 
at  the  tips  of  the  digits  as  described 
by  Zander  has  not  yet  been  confirmed 
by  later  observers,  but  the  migration 
of  the  areas  to  the  dorsal  surface  neces- 
sitated by  such  a  location  of  the  primary 
differentiation  affords  an  explanation 
of  the  otherwise  anomalous  cutaneous 
nerve-supply  of  the  nail-areas  in  the 
adult,  this  being  from  the  palmar 
(plantar)   nerves. 

The  Development  of  the  Hairs. 

— The    hairs   begin   to    develop    at 

about  the  third  month  and  continue   Fig.    82.— Longitudinal 

.  .    .  Section  through  the 

to  be  formed  during  the  remammg      nail  Area  in  an  Em- 

portions  of  fetal  life.    They  arise  as      ^^^^'^  °^  ^7  cm. 

...         ,.,.,,  ^,  cp,  Eponj'chium ;   n,  nail 

solid  cyhndrical  downgrowths,  pro-       substance;     nb,     nail 

lectins:  obliquely  into  the  subjacent      '^^^'  •^^'  stratum  cor- 
•'  °  ^        -^  .  ^        neum ;  sp,  sole  plate. — 

dermis   from   the  lower  surface  of       (Okamura.) 


DEVELOPMENT    OF    THE    HAIRS. 


153 


the  epidermis.  As  these  dov/ngrowths  continue  to  elon- 
gate, they  assume  a  somewhat  club-shaped  form  (Fig. 
83),  and  later  the  extremity  of  each  club  moulds  itself 
over  the  summit  of  a  small  papilla  which  develops  from 
the  dermis  (Fig.  83).  Even  before  the  dermal  papilla 
has  made  its  appearance,  however,  a  differentiation  of 


TJl 


Fig.  83. — The  Development  of  a  Hair. 
c,  Cylindrical  cells  of  stratum  mucosum ;   /;/,  wall  of  hair  follicle ;  m, 
mesoderm ;  nm,  stratum  mucosum  of  epidermis ;  p,  hair  papilla ;  r, 
root  of  hair;  s,  sebaceous  gland. — (Kollmann.) 


the  cells  of  the  downgrowth  becomes  evident,  the  cen- 
tral cells  becoming  at  first  spindle-shaped  and  then  un- 
dergoing a  keratinization  to  form  the  hair  shaft,  while 
the  more  peripheral  ones  assume  a  cuboidal  form  and 
constitute  the  lining  of  the  hair  follicle.     The  further 


154  DEVELOPMENT    OF    THE    HAIRS. 

growth  of  the  hair  takes  place  by  the  addition  to  its 
basal  portion  of  new  keratinized  cells,  probably  produced 
by  the  multiplication  of  the  epidermal  cells  which  en- 
velop the  papilla. 

From  the  cells  which  form  the  lining  of  each  follicle  an 
outgrowth  takes  place  into  the  surrounding  dermis  to  form 
a  sebaceous  gland,  which  is  at  first  solid  and  club-shaped, 
though  later  it  becomes  lobed.  The  central  cells  of  the  out- 
growth separate  from  the  peripheral  and  from  one  another, 
and,  their  protoplasm  undergoing  a  fatty  degeneration,  they 
finally  pass  out  into  the  space  between  the  follicle  walls  and 
the  hair  and  so  reach  the  surface,  the  peripheral  cells  later 
giving  rise  by  division  to  new  generations  of  central  cells. 
During  fetal  life  the  fatty  material  thus  poured  out  upon 
the  surface  of  the  body  becomes  mingled  with  the  cast-off 
epitrichial  cells  and  constitutes  the  white  oleaginous  sub- 
stance, the  vcrnix  caseosa,  which  covers  the  surface  of  the 
new-born  child.  The  muscles,  arrectores  pilonmi,  connected 
with  the  hair  follicles  arise  from  the  mesenchyme  cells  of 
the  surrounding  dermis. 

The  first  growth  of  hairs  forms  a  dense  covering  over  the 
entire  surface  of  the  fetus,  the  hairs  which  compose  it  being 
exceedingly  fine  and  silky  and  constituting  what  is  termed 
the  lanugo.  This  growth  is  cast  off  soon  after  birth,  except 
over  the  face,  where  it  is  hardly  noticeable  on  account  of  its 
extreme  fineness  and  lack  of  coloration.  The  coarser  hairs 
which  replace  it  in  certain  regions  of  the  body  probably 
arise  from  new  follicles,  since  the  formation  of  follicles 
takes  place  throughout  the  later  periods  of  fetal  life  and 
possibly  after  birth.  But  even  these  later  formed  hairs  do 
not  individually  persist  for  any  great  length  of  time,  but 
are  continually  being  shed,  new  or  secondary  hairs  normally 
developing  in  their  places.  The  shedding  of  a  hair  is  pre- 
ceded by  a  cessation  of  the  proliferation  of  the  cells  cover- 


DEVELOPMENT    OF    THE    SUDORIPAROUS    GLANDS. 


155 


ing-  the  dermal  papilla  and  by  a  shrinkage  of  the  papilla, 
whereby  it  becomes  detached  from  the  hair,  and  the  replac- 
ing hair  arises  from  a  papilla  which  is  probably  budded 
off  from  the  older  one  before  its  degeneration  and  carries 
with  it  a  cap  of  epidermal  cells. 

It  is  uncertain  whether 
the  cases  of  excessive  de- 
velopment of  hair  over  the 
face  and  upper  part  of  the 
body  which  occasionally  oc- 
cur are  due  to  an  excessive 
development  of  the  later 
hair  follicles  (hypertricho- 
sis) or  to  a  persistence  and 
continued  growth  of  the 
lanueo. 


— h 


Fig.  84. — Lower  Surface  of  a  De- 
tached Portion  of  Epidermis 
FROM  THE  Dorsum  of  the  Hand. 

h,  Hair  follicle ;  s,  sudoriparous 
gland. —  (Blaschko.) 


The  Development 
of  the  Sudoriparous 
Glands. — The  sudoripar- 
ous glands  arise  during 
the  fifth   month   as   solid 

cylindrical  outgrowths  from  the  primary  ridges  of  the  epi- 
dermis (Fig.  84),  and  at  first  project  vertically  downward 
into  the  subjacent  dermis.  Later,  however,  the  lower  end 
of  each  downgrowth  is  thrown  into  coils,  and  at  the  same 
time  a  lumen  appears  in  the  center.  Since,  however,  the 
cylinders  are  formed  from  the  deeper  layers  of  the  epider- 
mis, their  lumina  do  not  at  first  open  upon  the  surface,  but 
gradually  approach  it  as  the  cells  of  the  deeper  layers  of  the 
epidermis  replace  those  which  are  continually  being  cast  off 
from  the  surface  of  the  stratum  conieum.  The  final  open- 
ing to  the  surface  occurs  during  the  seventh  month  of 
development. 

The  Development  of  the  Mammary  Glands. — In  the 
majority  of  the  lower  mammals  a  number  of  mammary 
glands  occur,  arranged  in  two  longitudinal  rows,  and  it  has 


156 


DEVELOPMENT    OF    THE    MAMMARY    GLANDS. 


been  observed  that  in  the  pig-  the  first  indication  of  their 
development  is  seen  in  a  thickening  of  the  epidermis  along 
a  line  situated  at  the  junction  of  the  abdominal  walls  with 
the  membrana  reuniens  (Schulze).  This  thickening  subse- 
quently becomes  a  pronounced  ridge,  the  milk  ridge,  from 
which,  at  certain  points,  the  mammary  glands  develop,  the 

ridge    disappearing    in 
'^"N^  the    intervals.       In    a 

human  embryo  4  mm. 
in  length  an  epidermal 
thickening  has  been  ob- 
served which  extended 
from  just  below  the 
axilla  to  the  inguinal 
,.  region  (Fig.  85)  and 
was  apparently  equiva- 
lent to  the  milk  line  of 
the  pig,  and  in  em- 
bryos of  14  or  15  mm. 

the   upper   end    of   the 
Fig.  85. — Milk  Ridge  (mr)  IN  A  Human      ...       ,      .  , 

Embryo.— {Kallius.)  bne  had  become  a  pro- 

nounced    ridge,     while 
more  posteriorly  the  thickening  had  disappeared. 

The  further  history  of  the  ridge  has  not,  however,  been 
yet  traced  in  human  embryos,  and  the  next  stage  of  the 
development  of  the  glands  which  has  been  observed  is  one 
in  which  they  are  represented  by  a  circular  thickening  of 
the  epidermis  which  projects  downward  into  the  dermis 
(Fig.  86,  A).  Later  the  thickening  becomes  lobed  (Fig. 
86,  B),  and  its  superficial  and  central  cells  become  corni- 
fied  and  are  cast  off,  so  that  the  gland  area  appears  as  a 
depression  of  the  surface  of  the  skin.  During  the  fifth  and 
sixth  months  the  lobes  elongate  into  solid  cylindrical  col- 
umns of  cells  (Fig.  87)  resembling  not  a  little  the  cylinders 


DEVELOPMENT    OF    THE    MAMMARY    GLANDS.  157 

which  become  converted  into  sudoriparous  glands,  and  each 
column  becomes  slightly  enlarged  at  its  lower  end,  from 
which  outgrowths  begin  to  develop  to  form  the  acini.  A 
lumen  first  appears  in  the  lower  ends  of  the  columns  and 
is  formed  by  the  separation  and  breaking  down  of  the  cen- 

^/■^  »  e  «  o  o  o  ''"'^■' o  .     - 

,  ' ,  -  '  '.-•>.i'g''T  ■  "  '■=  -  "  *•  <♦  * ."  "•'"V  ' '  '  '  '  ■ 


B 


Fig.  86. — Sections  through  the  Epidermal  Thickenings  which  Rep- 
resent THE  Mammary  Gland  in  Embryos  (A)  of  6  cm.  and  (5) 

OF    10.2    CM. 

tral  cells,  the  peripheral  cells  persisting  as  the  lining  of  the 
acini  and  ducts. 

The  elevation  of  the  gland  area  above  the  surface  to  form 
the  nipple  appears  to  occur  at  different  periods  in  different 
embryos  and  frequently  does  not  take  place  until  after  birth. 
In  the  region  around  the  nipple  sudoriparous  and  sebaceous 
glands  develop,  the  latter  also  occurring  within  the  nipple 


158  DEVELOPMENT    OF    THE    MAMMARY    GLANDS. 

area  and  frequently  opening  into  the  extremities  of  the 
lacteal  ducts.  In  the  areola,  as  the  area  surrounding  the 
nipple  is  termed,  other  glands  known  as  Montgomery's 
glands,  also  appear,  their  development  resembling  that  of 
the  mammary  gland  so  closely  as  to  render  it  probable  that 
they  are  really  rudimentary  mammary  glands. 

The  further  development  of  the  glands,  consisting  of  an 
increase  in  the  length  of  the  ducts  and  the  development  from 
them  of  additional  acini,  continues  slowly  up  to  the  time  of 
puberty  in  both  sexes,  but  at  that  period  further  growth 


Fig.  87. — Section  through  the  Mammary  Gland  of  an  Embryo  of 
25  cm.     I,   Stroma  of  the  gland. —  {From  Nagcl,  after  Basch.) 

ceases  in  the  male,  while  in  females  it  continues  for  a  time 
and  the  subjacent  dermal  tissues,  especially  the  adipose  tis- 
sue, undergo  a  rapid  development. 

The  occurrence  of  a  milk  ridge  has  not  yet  been  observed 
in  a  sufficient  number  of  embryos  to  determine  whether  it  is 
a  normal  development  or  is  associated  with  the  formation  of 
supernumerary  glands  {polymastia).  This  is  by  no  means  an 
infrequent  anomaly;  it  has  been  observed  in  19  per  cent,  of 
over  100,000  soldiers  of  the  German  army  who  were  examined, 
and  occurs  in  47  per  cent,  of  individuals  in  certain  regions  of 
Germany.  The  extent  to  which  the  anomaly  is  developed 
varies  from  the  occurrence  of  well-developed  accessory  glands 
to  that  of  rudimentary  accessory  nipples  (hyperthclia) ,  these 
latter  sometimes  occurring  in  the  areolar  area  of  a  normal 
gland  and  being  possibly  due  in  such  cases  to  an  hypertrophy 
of  one  or  more  of  Montgomery's  glands. 


LITERATURE.  I  59 

Although  the  mammary  glands  are  typically  functional  only 
in  females  in  the  period  immediately  succeeding  pregnancy, 
cases  are  not  unknown  in  which  the  glands  have  been  well 
developed  and  functional  in  males  {gyncEcomastia) .  Further- 
more, a  functional  activity  of  the  glands  normally  occurs  imme- 
diately after  birth,  infants  of  both  sexes  yielding  a  few  drops 
of  a  milky  fluid,  the  so-called  zvitch-milk  (Hexenmilch),  when 
the  glands  are  subjected  to  pressure. 

LITERATURE. 

J.  T.  BowEN :  "  The  Epitrichial  Layer  of  the  Human  Epidermis,"  Anat. 
Anzeiger,  iv,  1889. 

Brouha  :  "  Recherches  stir  les  diverses  phases  du  developpement  et 
de  I'activite  de  la  mammelle,"  Arch,  de  Biol.,  xxi,  1905. 

G.  BuRCKHARD :  "  Ueber  embryonale  Hypermastie  und  HypertheUe," 
Anat.  Hefte,  viii,  1897. 

H.  Head  :  "  On  Disturbances  of  Sensation  with  Special  Reference  to  the 
Pain  of  Visceral  Disease,"  Brain,  xvi,  1892;  xvii,  1894;  and  xix, 
1896. 

E.  Kallius  :  "  Ein  Fall  von  Milchleiste  bei  einem  menschhchen  Em- 
bryo," Anat.  Hefte,  viii,  1897. 

T.  Okamura:  "Ueber  die  Entwicklung  des  Nagels  beim  Menschen," 
Archiv  filr  Dermatol,  und  SyphiloL,  xxv,  1900. 

H.  Schmidt:  "Ueber  normale  Hyperthelie  menschlicher  Embryonen 
und  iiber  die  erste  Anlage  der  menschhchen  Milchdriisen  iiber- 
haupt,"  Morphol.  Arbeiten,  xvii,  1897. 

C.  S.  Sherrington  :  "  Experiments  in  Examination  of  the  Peripheral 
Distribution  of  the  Fibres  of  the  Posterior  Roots  of  some  Spinal 
Nerves,"  Philos.  Trans.  Royal  Soc,  clxxxiv,  1^3,  and  cxc,  1898. 

P.  Stohr  :  "  Entwickelungsgeschichte  des  menschhchen  Wollhaares," 
Aiiat.  Hefte,  xxiii,  1903. 

H.  Strahl  :  "  Die  erste  Entwicklung  der  Mammarorgane  beim  Men- 
schen," Verhandl.  Anat.  Gcsellsch.,  xn,  1898. 


CHAPTER    VII. 

THE   DEVELOPMENT   OF   THE   CONNECTIVE 
TISSUES  AND  SKELETON. 

It  has  been  seen  that  the  cells  of  a  very  considerable  por- 
tion of  the  somatic  and  splanchnic  mesoderm,  as  well  as 
of  parts  of  the  mesodermic  somites,  become  converted  into 
mesenchyme.  A  very  considerable  portion  of  this  becomes 
converted  into  what  are  termed  connective  or  supporting" 
tissues,  characterized  by  consisting  of  a  non-cellular  matrix 
in  which  more  or  less  scattered  cells  are  embedded.  These 
tissues  enter  to  a  greater  or  less  extent  into  the  formation 
of  all  the  organs  of  the  body,  with  the  exception  of  those 
forming  the  central  nervous  system,  and  constitute  a  net- 
work which  holds  together  and  supports  the  elements  of 
which  the  organs  are  composed;  in  addition,  they  take  the 
form  of  definite  membranes  (serous  membranes,  fasciae), 
cords  (tendons,  ligaments),  or  solid  masses  (cartilage),  or 
form  looser  masses  or  layers  of  a  somewhat  spongy  texture 
(areolar  tissue).  The  intermediate  substance  is  somewhat 
varied  in  character,  being  composed  sometimes  of  white, 
non-branching,  non-elastic  fibers,  sometimes  of  yellow, 
branching,  elastic  fibers ;  of  white,  branching,  but  inelastic 
fibers  which  form  a  reticulum,  or  of  a  soft  gelatinous  sub- 
stance containing  considerable  quantities  of  mucin,  as  in  the 
tissue  which  constitutes  the  Whartonian  jelly  of  the  umbili- 
cal cord.  Again,  in  cartilage  the  matrix  is  compact  and 
homogeneous,  or,  in  other  cases,  more  or  less  fibrous,  pass- 
ing over  into  ordinary  fibrous  tissue,  and,  finally,  in  bone  the 
organic  matrix  is  largely  impregnated  with  salts  of  lime. 

1 60 


DEVELOPMENT    OF    CONNECTIVE    TISSUE. 


i6i 


Two  views  exist  as  to  the  mode  of  formation  of  the 
matrix,  some  authors  maintaining  that  in  the  fibrous  tissues 
it  is  produced  by  the  actual  transformation  of  the  mesen- 
chyme cells  into  fibers,  while  others  claim  that  it  is  manu- 
factured by  the  cells  but  does  not  directly  represent  the  cells 
themselves.  Fibrils  and  material  out  of  which  fibrils  could 
be  formed  have  undoubtedly  been  observed  in  connective- 
tissue  cells,  but  whether  or  not  these  are  later  passed  to  the 
exterior  of  the  cell  to  form  a  connective-tissue  fiber  is  not 
yet  certain,  and  on  this  hangs  mainly  the  difference  between 


^u 


OX  '  { 


Fig.  88. — Portion  of  the  Center  of  Ossification  of  the  Parietal 
Bone  of  a  Human  Embryo. 


the  theories.  Recently  it  has  been  held  (Mall)  that  the 
mesenchyme  of  the  embryo  is  really  a  syncytium  in  and 
from  the  protoplasm  of  which  the  matrix  forms;  if  this  be 
correct,  the  distinction  which  the  older  views  make  between 
the  intercellular  and  intracellular  origin  of  the  matrix  be- 
comes of  little  importance. 

Bone  differs  from  the  other  varieties  of  connective  tissue 
in  that  it  is  never  a  primary  formation,  but  is  always  devel- 
oped either  in  fibrous  tissue  or  cartilage;  and  according  as 
it  is  associated  with  the  one  or  the  other,  it  is  spoken  of  as 
15 


t62 


DEVELOPMENT    OF    BONE. 


nienibrane  bone  or  cartilage  bone.  In  the  development  of 
membrane  bone  some  of  the  connective-tissue  ceUs,  which 
in  consequence  become  known  as  osteoblasts^  deposit  Hme 
saUs  in  the  matrix  in  the  form  of  bony  spicules  which  in- 
crease in  size  and  soon  unite  to  form  a  network  (Fig.  88). 
The  trabeculae  of  the  network  continue  to  thicken,  while,  at 
■  the  same  time,  the  forma- 

tion of  spicules  extends 
further  out  into  the  con- 
nective-tissue membrane, 
radiating  in  all  directions 
from  the  region  in  which 
it  first  developed.  Later 
the  connective  tissue 
which  lies  upon  either 
surface  of  the  reticular 
plate  of  bone  thus  pro- 
duced condenses  to  form 
a  stout  membrane,  the 
periosteum,  between 
which  and  the  osseous 
plate  osteoblasts  arrange 
themselves  in  a  more  or 
less  definite  layer  and 
deposit  upon  the  surface 
of  the  plate  a  lamella  of 
compact  bone.  A  mem- 
brane bone,  such  as  one 
of  the  flat  bones  of  the  skull,  thus  comes  to  be  composed  of 
two  plates  of  compact  bone,  the  inner  and  outer  tables,  en- 
closing and  united  to  a  middle  plate  of  spongy  bone  which 
constitutes  the  diploe. 

With  bones  formed  from  cartilage  the  process  is  some- 
what different.     In  the  center  of  the  cartilage  the  inter- 


FiG.  89. — Longitudinal  Section  of 
Phalanx  of  a  Finger  of  an  Em- 
bryo OF  32  Months. 

c,  Cartilage  trabeculae;  p,  periosteal 
bone;  po,  periosteum;  x,  ossifica- 
tion   center. — (Ssymonoivics.) 


DEVELOPMENT    OF    BONE. 


163 


po 


pi 


cellular  matrix  becomes  increased  so  that  the  cells  appear 
to  be  more  scattered  and  a  calcareous  deposit  forms  in  it. 
All  around  this  region  of  calcification  the  cells  arrange  them- 
selves in  rows  (Fig.  89)  and  the  process  of  calcification 
extends  into  the  trabeculse  of  matrix  which  separate  these 
rows.  While  these  processes  have  been  taking  place  the 
mesenchyme  surrounding 
the  cartilage  has  become  cc 
converted  into  a  periosteum 
(/jo),  similar  to  that  of 
membrane  bone,  and  its 
osteoblasts  deposit  a  layer 
of  bone  (/>)  upon  the  sur- 
face of  the  cartilage.  The 
cartilage  cells  now  disap- 
pear from  the  intervals  be- 
tween the  trabeculse  of  cal- 
cified matrix,  which  form 
a  fine  network  into  which 
masses  of  mesenchyme 
(Fig-.  90,  pi),  containing 
blood-vessels  and  osteo- 
blasts, here  and  there  pene- 
trate from  the  periosteum, 
after  having  broken  through 
the  layer  of  periosteal  bone. 
These  masses  absorb  a  portion  of  the  fine  calcified  network 
and  so  transform  it  into  a  coarse  network,  the  meshes  .of 
which  they  occupy  to  form  the  hone  marrow  (m),  and  the 
osteoblasts  which  they  contain  arrange  themselves  on  the  sur- 
face of  the  persisting  trabeculje  and  deposit  layers  of  bone 
upon  their  surfaces.  In  the  meantime  the  calcification  of  the 
cartilage  matrix  has  been  extending,  and  as  fast  as  the  net- 
work of  calcified  trabeculse  is  formed  it  is  invaded  by  the 


Fig.  90. — The  Ossification  Center 
OF  Fig.  88  More  Highly  Magni- 
fied. 

c.  Ossifying  trabectilse;  cc,  cavity  of 
cartilage  network;  m,  marrow 
cells ;  p,  periosteal  bone ;  pi,  ir- 
ruption of  periosteal  tissue;  po, 
periosteum. —  (Ssymonozvics.) 


164  DEVELOPMENT    OF    BONE. 

mesenchyme,  until  finally  the  cartilage  becomes  entirely  con- 
verted into  a  mass  of  spongy  bone  enclosed  within  a  layer  of 
more  compact  periosteal  bone. 

As  a  rule,  each  cartilage  bone  is  developed  from  a  single 
center  of  ossification,  and  when  it  is  found  that  a  bone  of 
the  skull,  for  instance,  develops  by  several  centers,  it  is 
to  be  regarded  as  formed  by  the  fusion  of  several  primarily 
distinct  bones,  a  conclusion  which  may  generally  be  con- 
firmed by  a  comparison  of  the  bone  in  question  with  its 
homologues  in  the  lower  vertebrates.  Exceptions  to  this 
rule  occur  in  bones  situated  in  the  median  line  of  the  body, 
these  occasionally  developing  from  two  centers  lying  one 
on  either  side  of  the  median  line,  but  such  centers  are  usually 
to  be  regarded  as  a  double  center  rather  than  as  two  distinct 
centers,  and  are  merely  an  expression  of  the  fundamental 
bilaterality  which  exists  even  in  median  structures. 

More  striking  exceptions  are  to  be  found  in  the  long 
bones  in  which  one  or  both  extremities  develop  from  special 
centers  which  give  rise  to  the  epiphyses  (Fig.  91,  ep,  ep'), 
the  shaft  or  diaphysis  (d)  being  formed  from  the  primary 
center.  Similar  secondary  centers  appear  in  marked  promi- 
nences on  bones  to  which  powerful  muscles  are  attached 
(Fig.  91,  a  and  b),  but  these,  as  well  as  the  epiphysial  cen- 
ters, can  readily  be  recognized  as  secondary  from  the  fact 
that  they  do  not  appear  until  much  later  than  the  primary 
centers  of  the  bones  to  which  they  belong.  These  secondary 
centers  give  the  necessary  firmness  required  for  articular 
surfaces  and  for  the  attachment  of  muscles  and,  at  the  same 
time,  make  provision  for  the  growth  in  length  of  the  bone, 
since  a  plate  of  cartilage  always  intervenes  between  the 
epiphyses  and  the  diaphysis.  This  cartilage  continues  to  be 
transformed  into  bone  on  both  its  surfaces  by  the  extension 
of  both  the  epiphysial  and  diaphysial  ossification  into  it,  and, 


DEVELOPMENT    OF    BONE. 


i6s 


at  the  same  time,  it  grows  in  thickness  with  equal  rapidity 
until  the  bone  reaches  its  required  length,  whereupon  the 
rapidity  of  the  growth  of  the  cartilage  diminishes  and  it 
gradually  becomes  completely  ossi- 
fied, uniting  together  the  epiphysis 
and  diaphysis. 

The  growth  in  thickness  of  the 
long  bones  is,  however,  an  entirely 
different  process,  and  is  due  to  the 
formation  of  new  layers  of  periosteal 
l^one  on  the  outside  of  those  already 
present.  But  in  connection  with 
this  process  an  absorption  of  bone 
also  takes  place.  A  section  through 
the  middle  of  the  shaft  of  a  hume- 
rus, for  example,  at  an  early  stage 
of  development  would  show  a 
peripheral  zone  of  compact  bone 
surrounding  a  core  of  spongy  bone, 
the  meshes  of  the  latter  being  occu- 
pied by  the  marrow  tissue.  A  sim- 
ilar section  of  an  adult  bone,  on  the 
other  hand,  would  show  only  the 
peripheral  compact  bone,  much 
thicker  than  before  and  enclosing  a 
large  marrow  cavity  in  which  no 
trace  of  spongy  bone  might  remain. 
The  difference  depends  on  the  fact 
that  as  the  periosteal  bone  increases 
in  thickness,  there  is  a  gradual 
absorption  of  the  spongy  bone  and  also  of  the  earlier  layers 
of  periosteal  bone,  this  absorption  being  carried  on  by  large 
multinucleated  cells,  termed  osteoclasts,  derived  from  the 
marrow  mesenchyme.     By  their  action  the  bone  is  enabled 


Fig.  91. — The  Ossifica- 
tion Centers  of  the 
Femur. 

a,  and  b,  Secondary  cen- 
ters for  the  great  and 
lesser  trochanters ;  d, 
diaphysis ;  ep,  upper 
and  cp\  lower  epiphj'- 
sis. —  (Tcstiit.) 


l66  DEVELOPMENT    OF    BONE, 

to  reach  its  requisite  diameter  and  strength,  without  becom- 
ing an  almost  soHd  and  unwieldy  mass  of  compact  bone. 

During  the  ossification  of  the  cartilaginous  trabeculae 
osteoblasts  become  enclosed  by  the  bony  substance,  the  cavi- 
ties in  which  they  lie  forming  the  lacunce  and  processes 
radiating  out  from  them,  the  canaUculi,  so  characteristic  of 
bone  tissue.  In  the  growth  of  periosteal  bone  not  only  do 
osteoblasts  become  enclosed,  but  blood-vessels  also,  the 
Haversian  canals  being  formed  in  this  way,  and  around 


Fig.  92. — A,  Transverse  Section  of  the  Femur  of  a  Pig  Killed 
AFTER  Having  Been  Fed  with  Madder  for  Four  Weeks;  B,  the 
Same  of  a  Pig  Killed  Two  Months  after  the  Cessation  of 
the  Madder  Feeding. 

The  heavy  black  Hne  represents  the  portion  of  bone  stained  by  the 
madder. — {After  Flour  ens.) 

these  lamelte  of  bone  are  deposited  by  the  enclosed  osteo- 
blasts to  form  Haversian  systems. 

That  the  absorption  of  periosteal  bone  takes  place  during 
growth  can  be  demonstrated  by  taking  advantage  of  the  fact 
that  the  coloring  substance  madder,  when  consumed  with  food, 
tinges  the  bone  being  formed  at  the  time  a  distinct  red.  In 
pigs  fed  with  madder  for  a  time  and  then  killed  a  section  of 
the  femur  shows  a  superficial  band  of  red  bone  (Fig.  92,  A), 
but  if  the  animals  be  allowed  to  live  for  one  or  two  months 
after  the  cessation  of  the  madder  feeding,  the  red  band  will  be 
found  to  be  covered  by  a  layer  of  white  bone  varying  in  thick- 
ness according  to  the  interval  elapsed  since  the  cessation  of 
feeding  (Fig.  92,  B)  ;  and  if  this  interval  amount  to  four 
months,  it  will  be  found  that  the  thickness  of  the  uncolored 


DEVELOPMENT  OF  THE  SKELETON. 


167 


bone  between  the  red  bone  and  the  marrow  cavity  will  have 
greatly  diminished  (Flourens). 

The  Development  of  the  Skeleton. — Embryologically 

considered,  the  skeleton  is  composed  of  two  portions,  the 
axial  skeleton,  consisting  of  the  sknll,  the  vertebrae,  ribs, 


Sep 


Sca. 


V^ 


^f 


Fig.  93. — Frontal  Section  through  Mesodermic  Somites  of  a  Calf 

Embryo. 
isa,  intersegmental  artery ;  my,  myotome ;  n,  central  nervous  system ; 

nc,    notochord;    sea    and   scp,    anterior   and   posterior   portions    of 

sclerotomes. 


and  sternmii,  developing  from  the  sclerotomes  of  the  meso- 
dermal somites,  and  the  appendicular  skeleton,  which  in- 
cludes the  pectoral  and  pelvic  girdles  and  the  bones  of  the 
limbs,  and  which  arises  from  the  mesenchyme  of  the  somatic 
mesoderm.     It  will  be  convenient  to  consider  first  the  devel- 


1 68 


DEVELOPMENT    OF    THE    VERTEBRAE, 


opment  of  the  axial  skeleton,  and  of  this  the  differentiation 
of  the  vertebral  column  and  ribs  may  first  be  discussed. 
The  Development  of  the  Vertebrae  and  Ribs. — The 

mesenchyme  formed  from  the  sclerotome  of  each  meso- 
dermic  somite  grows  inward  toward  the  median  line  and 
forms  a  mass  lying-  between  the  notochord  and  the  myotome, 
separated  from  the  similar  mass  in  front  and  behind  by  some 

loose  tissue  in  which  lies 
an  intersegmental  artery. 
Towards  the  end  of  the 
third  week  of  development 
the  cells  of  the  posterior 
portion  of  each  sclerotome 
w  condense  to  a  tissue  more 
compact  than  that  of  the 
anterior  portion  (Fig.  93), 
and  a  little  later  the  two 
portions  become  separated 
by  a  cleft.  At  about  the 
Fig.    94. — Transverse    Section    same     time     the     posterior 

THROUGH  the  INTERVERTEBRAL  .  , 

Plate     of     the     First     Cervical   portion      sends     a     process 
Vertebra  of  a   Calf  Embryo  of  medially,     to     enclose     the 

8.8   MM.  111  •    •  -1 

hc\  Intervertebral  plate;  m^  fourth  notOchord  by  unitmg  With 
myotome;  s,  hypochordal  bar;_Z/,  ^  corresponding  process 
spinal  accessory  nerve. —  (Froriep.)     .  ,  ,  ^ 

from  the  sclerotome  of 
the  other  side,  and  it  also  "  sends  a  prolongation  dorsally 
between  the  myotome  and  the  spinal  cord  to  form  the  verte- 
bral arch,  and  a  third  process  laterally  and  ventrally  along 
the  distal  border  of  the  myotome  to  form  a  costal  process 
(Fig.  94).  The  looser  tissue  of  the  anterior  half  of  the 
sclerotome  also  grows  medially  to  surround  the  notochord, 
filling  up  the  intervals  between  successive  denser  portions, 
and  it  forms  too  a  membrane  extending  between  successive 
vertebral  arches.     Later  the  tissue  surrounding  the  noto- 


DEVELOPMENT    OF    THE    VERTEBRA.  1 69 

chord  which  is  derived  from  the  anterior  half  of  the  sclero- 
tome associates  itself  with  the  posterior  portion  of  the  pre- 
ceding sclerotome  to  form  what  will  later  be  a  vertebra, 
the  tissue  occupying-  and  adjacent  to  the  line  of  division 
between  the  anterior  and  posterior  portions  of  the  sclero- 
tomes condensing  to  form  intervertebral  fibrocartilages. 
Consequently  each  vertebra  is  formed  by  parts  from  two 
sclerotomes,  the  original  intersegmental  artery  passes  over 
the  body  of  a  vertebra,  and  the  vertebrae  themselves  alter- 
nate with  the  myotomes.  With  this  differentiation  the  first 
or  blastemic  stage  of  the  development  of  the  vertebrae  closes. 
In  the  second  or  cartilag^inous  stage,  portions  of  the  scle- 
rotomic  mesenchyme  become  transformed  into  cartilage. 
In  the  posterior  portion  of  each  vertebral  body,  that  is  to 
say  in  the  portion  formed  from  the  anterior  halves  of  the 
more  posterior  of  the  two  pairs  of  sclerotomes  entering  into 
its  formation,  two  centers  of  chondrification  appear,  one  on 
each  side  of  the  median  line,  and  these  eventually  unite  to 
form  a  single  cartilaginous  body,  the  chondrification  prob- 
ably also  extending  to  some  extent  into  the  denser  anterior 
portion  of  the  body.  A  center  also  appears  in  each  half 
of  the  vertebral  arch  and  in  each  costal  process,  the  carti- 
lages foniied  in  the  costal  processes  of  the  anterior  cervical 
region  uniting  across  the  median  line  below  the  notochord, 
to  form  what  has  been  termed  a  hypochordal  bar  (Fig^s.  94 
and  95).  These  bars  are  for  the  most  part  but  transitory, 
recalling  structures  occurring  in  the  lower  vertebrates;  in 
the  mammalia  they  degenerate  before  the  close  of  the  carti- 
laginous stage  of  development,  except  in  the  case  of  the 
atlas,  whose  development  will  be  described  later.  As 
development  proceeds  the  cartilages  of  the  vertebral  arches 
and  costal  processes  increase  in  length  and  come  into  con- 
tact with  the  cartilaginous  bodies,  with  which  they  even- 
tually fuse,  and  from  the  vertebral  arches  processes  grow 
16 


I/O  DEVELOPMENT  OF  THE  VERTEBRA. 

out   which   represent    the    future   transverse   and   articular 
processes. 

The  fusion  of  the  cartilage  of  the  costal  process  with  the 
body  of  the  vertebra  does  not,  however,  persist.  Later  a 
solution  of  the  junction  occurs  and  the  process  becomes  a 
rib  cartilage,  the  mesenchyme  surrounding  the  area  of  solu- 
tion forming  the  costo-vertebral  ligaments.  At  first  the  rib 
cartilage  is  separated  by  a  distinct  interval  from  the  trans- 
verse process  of  the  vertebral  arch,  but  later  it  develops  a 
process,  the  tubercle,  which  bridges  the  gap  and  forms  an 
articulation  with  the  transverse  process. 

The  mesenchyme  which  extends  between  successive  ver- 
tebral arches  does  not  chondrify,  but  later  becomes  trans- 
formed into  the  interspinous  ligaments  and  the  ligamenta 
flava,  while  the  anterior  and  posterior  longitudinal  liga- 
ments are  formed  from  unchondrified  portions  of  the  tissue 
surrounding-  the  vertebral  bodies. 

As  was  pointed  out,  the  mesenchyme  in  the  region  of 
the  cleft  separating  the  anterior  and  posterior  portions  of  a 
sclerotome  becomes  an  intervertebral  fibrocartilage,  and,  as 
the  cartilaginous  bodies  develop,  the  portions  of  the  noto- 
chord  enclosed  by  them  become  constricted,  while  at  the 
same  time  the  portions  in  the  intervertebral  regions  increase 
in  size.  Finally  the  notochord  disappears  from  the  verte- 
bral regions,  although  a  canal,  representing  its  former  posi- 
tion, traverses  each  body  for  a  considerable  time,  and  in  the 
intervertebral  regions  it  persists  as  relatively  large  flat 
disks  forming  the  pulpy  nuclei  of  the  fibrocartilages. 

The  mode  of  development  described  above  applies  to  the 
great  majority  of  the  vertebrae,  but  some  departures  from 
it  occur,  and  these  may  be  conveniently  considered  before 
passing  on  to  an  account  of  the  ossification  of  the  carti- 
lages. The  variations  affect  principally  the  extremes  of  the 
series.     Thus  the  posterior  vertebrae  present  a  reduction  of 


DEVELOPMENT    OF    THE    VERTEBRA. 


171 


the  vertebral  arches,  those  of  the  last  sacral  vertebrae  being 
but  feebly  developed,  while  in  the  coccygeal  vertebra  they 
are  indicated  only  in  the  first.  In  the  first  cervical  vertebra, 
the  atlas,  the  reverse  is  the  case,  for  the  entire  adult  ver- 
tebra is  formed  from  the  posterior  portion  of  a  sclerotome, 
its  lateral  masses  and  posterior  arch  being  the  vertebral 


Fig.  95.— Longitudinal  Section  through  the  Occipital  Region  and 
Upper  Cervical  Vertebr.t:  of  a  Calf  Embryo  of  18.5  mm. 

bas.  Basilar  artery;  ch,  notochord ;  Kc^'\  vertebral  centra;  /r"*,  inter- 
vertebral disks;  occ,  basioccipital;  Sc^~^,  hypochordal  bars.— 
(Froriep.) 


arches,  while  its  anterior  arch  is  the  hypochordal  bar,  which 
persists  in  this  vertebra  only.  A  well-developed  centrum 
is  also  formed,  however  (Fig.  95),  but  it  does  not  unite 
with  the  parts  derived  from  the  preceding  sclerotome,  but 
during  its  ossification  unites  with  the  centrum  -of  the  epis- 
tropheus (axis),  forming  the  odontoid  process  of  that  ver- 


1/2  DEVELOPMENT    OF    THE    VERTEBRA    AND    RIBS, 

tebra.  The  epistropheus  consequently  is  formed  by  one  and 
a  half  sclerotomes,  while  but  half  a  one  constitutes  the  atlas. 

The  extent  to  which  the  ribs  are  developed  in  connec- 
tion with  the  various  vertebrae  also  varies  considerably. 
Throughout  the  cervical  region  they  are  short,  the  upper 
five  or  six  being  no  longer  than  the  transverse  processes, 
with  the  tips  of  which  their  extremities  unite  at  an  early 
stage.  In  the  upper  five  or  six  vertebrae  a  relatively  large 
interval  persists  between  the  rib  and  the  transverse  process, 
forming  the  vertebral  foramen,  throug'h  which  the  vertebral 
vessels  pass,  but  in  the  seventh  vertebra  the  fusion  is  more 
extensive  and  the  foramen  is  very  small  and  hardly  notice- 
able. In  the  thoracic  region  the  ribs  reach  their  greatest 
development,  the  upper  eight  or  nine  extending  almost  to 
the  mid-ventral  line,  where  their  extremities  unite  to  form 
a  longitudinal  cartilaginous  bar  from  which  the  sternum 
develops  (see  p.  175).  The  lower  three  or  four  thoracic 
ribs  are  successively  shorter,  however,  and  lead  to  the  con- 
dition found  in  the  lumbar  vertebrae,  where  they  are  again 
greatly  reduced  and  firmly  united  with  the  transverse  proc- 
esses, the  union  being  so  close  that  only  the  tips  of  the 
latter  can  be  distinguished,  forming  what  are  known  as  the 
accessory  tubercles.  In  the  sacral  region  the  ribs  are  re- 
duced to  short  flat  plates,  which  unite  together  to  form  the 
lateral  masses  of  the  sacrum,  and,  finally,  in  the  coccygeal 
region  the  blastemic  costal  processes  of  the  first  vertebra 
unite  with  the  transverse  processes  to  form  the  transverse 
processes  of  the  adult  vertebra,  but  no  indications  of  them 
are  to  be  found  in  the  other  vertebrae  beyond  the  blastemic 
stage. 

The  third  stage  in  the  development  of  the  axial  skeleton 
begins  with  the  ossification  of  the  cartilages,  and  in  each 
verteljra  there  are  typically  as  many  primary  centers  of 
ossification  as  there  were  originally  cartilages,  except  that 


DEVELOPMENT    OF    THE    VERTEBRA    AND    RIBS. 


^73 


but  a  single  center  appears  in  the  body.  Thus,  to  take  a 
thoracic  vertebra  as  a  type,  a  center  appears  in  each  half 
of  each  vertebral  arch  at  the  base  of  the  transverse  process 
and  gradually  extends  to  form  the  bony  lamina,  pedicle,  and 
the  greater  portion  of  the  transverse  and  spinous  processes ; 
a  single  center  gives  rise  to  the  body  of  the  vertebra;  and 
each  rib  ossifies  from  a  sing-le  center.  These  various  cen- 
ters appear  early  in  embryonic  life,  but  the  complete  trans- 
formation of  the  cartilages  into  bone  does  not  occur  until 


-■"  n 

\, 

Fig.  96. — A  J  A  Vertebra  at  Birth;  B,  Lumbar  Vertebra  showing 
Secondary  Centers  of  Ossification. 

a.  Center  for  the  articular  process;  c,  body;  el,  lower  epiphysial  plate; 
en,  upper  epiphysial  plate ;  na,  vertebral  arch ;  s,  center  for  spinous 
process;   t,  center  for  transverse  process. —  (Sappey.) 

some  time  after  birth,  each  vertebra  at  that  period  consist- 
ing of  three  parts,  a  body  and  two  halves  of  an  arch,  sepa- 
rated by  unossified  cartilage  (Fig.  96,  A).  At  about 
puberty  secondary  centers  make  their  appearance;  one  ap- 
pears in  the  cartilage  which  still  covers  the  anterior  and 
posterior  surfaces  of  the  vertebral  body,  producing  disks 
of  bone  in  these  situations,  another  appears  at  the  tip  of 
each  spinous  and  transverse  process  (Fig.  96,  B),  and  in 
the  lumbar  vertebrje  others  appear  at  the  tips  of  the  articu- 
lating processes.  The  epiphyses  so  formed  remain  separate 
until  growth  is  completed  and  between  the  sixteenth  and 
twenty-fifth  years  unite  with  the  bones  formed  from  the 
primary  centers,  which  have  fused  by  this  time,  to  form 
a  single  vertebra. 


174 


DEVELOPMENT    OF    THE    VERTEBRA    AND    RIBS. 


Each  rib  ossifies  from  a  single  primary  center  situated 
near  the  angle,  secondary  centers  appearing  for  the  capit- 
ulum  and  tuberosity. 

In  some  of  the  vertebrze  modifications  of  this  typical 
mode  of  ossification  occur.  Thus,  in  the  upper  five  cervi- 
cal vertebrae  the  centers  for  the  rudimentary  ribs  are  sup- 
pressed, ossification  extending  into  them  from  the  vertebral 
arch  centers,  and  a  similar  suppression  of  the  costal  centers 
occurs  in  the  lower  lumbar  vertebra,  the  first  only  develop- 
ing  a   separate   rib   center.     Furthermore,    in   the   atlas   a 


.Fig.  97. — A,  Upper  Surface  of  the  Fir.st  Sacral  Vertebra,  and  B, 
Ventral  View  of  the  Sacrum  showing  Primary  Centers  of 
Ossification. 

c,  Body;  na,  vertebral  arch;  r,  rib  center. — (Sappey.) 

double  center  appears  in  the  persisting  hypochordal  bar,  and 
the  body  which  corresponds  to  the  atlas,  after  developing 
the  terminal  epiphysial  disks,  fuses  with  the  body  of  the 
epistropheus  (axis)  to  form  its  odontoid  process,  this  ver- 
tebra consequently  possessing,  in  addition  to  the  typical  cen- 
ters, one  (double)  other  primary  and  two  secondary  cen- 
ters. In  the  sacral  region  the  typical  centers  appear  in  all 
five  vertebrae,  with  the  exception  of  rib  centers  for  the  last 
one  or  two  (Fig.  97)  and  two  additional  secondary  centers 


DEVELOPMENT    OF    THE    STERNUM.  1/5 

g-ive  rise  to  plate-like  epiphyses  on  each  side,  the  upper 
plates  forming  the  articular  surface  of  the  ilium.  At  about 
the  twenty-fifth  year  all  the  sacral  vertebrae  unite  to  form 
a  single  bone,  and  a  similar  fusion  occurs  also  in  the  rudi- 
mentary vertebrae  of  the  coccyx. 

The  majority  of  the  anomalies  seen  in  the  vertebral  column 
are  due  to  the  imperfect  development  of  one  or  more  cartilages 
or  of  the  centers  of  ossification.  Thus,  a  failure  of  an  arch  to 
unite  with  the  body  or  even  the  complete  absence  of  an  arch 
or  half  an  arch  may  occur,  and  in  cases  of  spina  bifida  the  two 
halves  of  the  arches  fail  to  unite  dorsally.  Occasionally  the 
two  parts  of  the  double  cartilaginous  center  for  the  body  fail 
to  unite,  a  double  body  resulting ;  or  one  of  the  two  parts 
may  entirely  fail,  the  result  being  the  formation  of  only  one- 
half  of  the  body  of  the  vertebra.  Other  anomalies  result  from 
the  excessive  development  of  parts.  Thus,  the  rib  of  the 
seventh  cervical  vertebra  may  sometimes  remain  distinct  and 
be  long  enough  to  reach  the  sternum,  and  the  first  lumbar  rib 
may  also  fail  to  unite  with  its  vertebra.  On  the  other  hand, 
the  first  thoracic  rib  is  occasionally  found  to  be  imperfect. 

The  Development  of  the  Sternum. — Longitudinal  bars, 
which  are  formed  by  the  fusion  of  the  ventral  ends  of  the 
anterior  eight  or  nine  cartilaginous  thoracic  ribs,  represent 
the  future  sternum.  At  an  early  period  the  two  bars  come 
into  contact  anteriorly  and  fuse  together  (Fig.  98),  and  at 
this  anterior  end  two  usually  indistinctly  separated  masses 
of  cartilage  are  to  be  observed  at  the  vicinity  of  the  points 
where  the  ventral  ends  of  the  cartilaginous  clavicles  articu- 
late. These  are  the  cpistcrnal  cartilages  {em),  which  later 
normally  unite  with  the  longitudinal  bars  and  form  part 
of  the  manubrium  sterni,  though  occasionally  they  persist 
and  ossify  to  form  the  ossa  suprasternalia.  The  fusion  of 
the  longitudinal  bars  gradually  extends  backward  until  a 
single  elongated  plate  of  cartilage  results,  with  which  the 
seven  anterior  ribs  are  united,  one  or  two  of  the  more  pos- 
terior ribs  which  originally  took  part  in  the  formation  of 
each    bar   having    separated.     The    portions    of    the    bars 


176  DEVELOPMENT    OF    THE    STERNUM. 

formed  by  these  posterior  ribs  constitute  the  xiphoid  process. 
The  ossification  of  the  sternum  (Fig.  99)  partakes  to  a 
certain  extent  of  the  original  bilateral  segmental  origin  of 
the  cartilage,  but  there  is  a  marked  condensation  of  the 
centers  of  ossification  and  considerable  variation  in  their 
number  also  occurs.  In  the  portion  of  the  cartilage  which 
lies  below  the  junction  of  the  third  costal  cartilages  a  series 


'sS.-'V  ' 


'^ 


Fig.  98. — Formation  of  the  Sternum  in  an  Embryo  of  about  3  cm. 
elj   Clavicle;    cm,   episternal    cartilage. —  (Rtige.) 

of  pairs  of  centers  appears  just  about  birth,  each  center 
probably  representing  an  epiphysial  center  of  a  correspond- 
ing rib.  Later  the  centers  of  each  pair  fuse  and  the  single 
centers  so  formed,  extending  throug'h  the  cartilage,  even- 
tually unite  to  form  the  greater  part  of  the  body  of  the 
bone.  In  each  of  the  two  uppermost  segments,  however, 
but  a  single  center  appears,  that  of  the  lower  segment  unit- 
ing with  the  more  posterior  centers  and  forming  the  upper 


DEVELOPMENT    OF    THE    STERNUM. 


177 


part  of  the  body,  while  the  uppermost  center  gives  rise  to 
the  manubrium,  which  frequently  persists  as  a  distinct  bone 
united  to  the  body  by  a  hinge-joint. 

A  failure  of  the  cartilaginous  bars  to  fuse  produces  the  con- 
dition known  as  cleft  sternum,  or  if  the  failure  to  fuse  affects 
only  a  portion  of  the  bars  there  results  a  perforated  sternum. 
A  perforation  or  notching  of  the  xiphoid  cartilage  is  of  fre- 


FiG.  99. — Sternum  of 
New-born  Child, 
SHOWING  Centers  of 
Ossification, 

/  to  VII,  Costal  carti- 
lages.—  {Gegenbaur.) 


Fig.  100. — Reconstruction  of  the 
Chondrocranium   of  an   Embryo  of 

14    MM. 

as,  Alisphenoid ;  bo,  basioccipital;  bs, 
basisphenoid ;  eo,  exoccipital ;  m, 
Meckel's  cartilage  ;  os,  orbitosphenoid ; 
p,  periotic;  ps,  presphenoid;  so,  sella 
turcica;   s,  supraoccipital. —  (Levi.) 


quent  occurrence  owing  to  this  being  the  region  where  the 
fusion  of  the  bars  takes  place  last. 

The  suprasternal  bones  are  the  rudiments  of  a  bone  or  carti- 
lage, the  omosternum,  situated  in  front  of  the  manubrium  in 
many  of  the  lower  mammalia.  It  furnishes  the  articular  sur- 
faces for  the  clavicles  and  is  possibly  formed  by  a  fusion  of  the 
ventral  ends  of  the  cartilages  which  represent  those  bones ; 
hence  its  appearance  as  a  pair  of  bones  in  the  rudimentary 
condition. 

The  Development  of  the  Skull. — In  its  earliest  stages 
the  human  skull  is  represented  by  a  continuous  mass  of 


1/8  DEVELOPMENT    OF    THE    SKULL. 

mesenchyme  which  invests  the  anterior  portion  of  the  noto- 
chord  and  extends  forward  beyond  its  extremity  into  the 
nasal  region,  forming  a  core  for  the  nasal  process  (see 
p.  88).  From  each  side  of  this  basal  mass  a  wing  projects 
dorsally  to  enclose  the  anterior  portion  of  the  medullary 
canal  which  will  later  become  the  cerebral  part  of  the  cen- 
tral nervous  system.  No  indications  of  a  segmental  ori- 
gin are  to  be  found  in  this  mesenchyme;  as  stated,  it  is  a 
continuous  mass,  and  this  is  likewise  true  of  the  cartilage 
which  later  develops  in  it. 

The  chondrification  occurs  first  along  the  median  line 
in  what  will  be  the  occipital  and  sphenoidal  regions  of  the 
skull  (Fig.  lOo)  and  thence  gradually  extends  forward  into 
the  ethmoidal  region  and  to  a  certain  extent  dorsally  at 
the  sides  and  behind  into  the  regions  later  occupied  by  the 
wings  of  the  sphenoid  {as  and  os)  and  the  squamous  por- 
tion of  the  occipital  (s).  No  cartilage  develops,  however, 
in  the  rest  of  the  sides  or  in  the  roof  of  the  skull,  but  the 
mesenchyme  of  these  regions  becomes  converted  into  a 
dense  membrane  of  connective  tissue.  While  the  chondri- 
fication is  proceeding  in  the  regions  mentioned,  the  mesen- 
chyme which  encloses  the  internal  ear  becomes  converted 
into  cartilage,  forming  a  mass,  the  periotic  capsule  (Fig. 
loo,  p),  wedged  in  on  either  side  between  the  occipital  and 
sphenoidal  regions,  with  which  it  eventually  unites  to  form 
a  continuous  chondrucraniurn,  perforated  l3y  foramina  for 
the  passage  of  nerves  and  vessels. 

The  posterior  part  of  the  basilar  portion  of  the  occipital 
cartilage  presents  certain  peculiarities  of  development.  In 
calf  embryos  there  are  in  this  region,  in  very  early  stages, 
four  separate  condensations  of  mesoderm  corresponding  to 
as  many  mesodermic  somites  and  to  the  three  roots  of  the 
hypoglossal  nerve  together  with  the  first  cervical  or  sub- 
occipital nerve    (Froriep)    (Fig.    loi).     These  mesenchy- 


DEVELOPMENT    OF    THE    SKULL. 


179 


mal  masses  in  their  gen- 
eral characters  and  rela- 
tions resemble  vertebral 
bodies,  and  there  are  good 
reasons  for  believing  that 
they  represent  four  verte- 
brae which,  in  later  stages, 
are  taken  up  into  the  skull 
region  and  fuse  with  the 
primitive  chondrocranium. 
In  the  human  embryo  they 
are  less  distinct  than  in 
lower  mammals,  but  since 
a  three-rooted  hypoglossal 
and  a  suboccipital  nerve 
also  occur  in  man  it  is 
probable  that  the  corre- 
sponding vertebrae  are  also 
represented.  Indeed,  con- 
firmation of  their  existence 
may  be  found  in  the  fact 
that  during  the  cartilagi- 
nous stage  of  the  skull  the 
hypoglossal  foramina  are 
divided  into  three  portions 
by  two  cartilaginous  parti- 
tions which  separate  the 
three  roots  of  the  hypo- 
glossal nerve.  It  seems 
certain  from  the  evidence 
derived  from  embryology 
and  comparative  anatomy 
that  the  human  skull  is 
composed    of    a    primitive 


J^.^ 


..«r-.-?§>>5.,j  _ 


Fig.  ioi. — Frontal  Section  through 
THE  Occipital  and  Upper  Cervi- 
cal Regions  of  a  Calf  Embryo  of 

8.7  MM. 

ai  and  ai^,  Intervertebral  arteries ;  bc^, 
first  cervical  intervertebral  plate; 
bo,  suboccipital  intervertebral  plate ; 
c^~',  cervical  nerves  ;  ch,  notochord ; 
K,  vertebral  centrum ;  m^'^,  occipi- 
tal myotomes ;  m'^^,  cervical  myo- 
tomes ;  o^~^  roots  of  hypoglossal 
nerve;  vj,  jugular  vein;  .v  and  xi, 
vagus  and  spinal  accessory  nerves. 
—  (Froricp.) 


l8o  DEVELOPMENT    OF    THE    SKULL. 

unsegmental  chondrocraninm  plus  four  vertebrae,  the  latter 
being  added  to  and  incorporated  with  the  occipital  portion 
of  the  chondrocraninm. 

Emphasis  must  be  laid  upon  the  fact  that  the  cartilagi- 
nous portion  of  the  skull  forms  only  the  base  and  lower 
portions  of  the  sides  of  the  cranium,  its  entire  roof,  as  well 
as  the  face  region,  showing  no  indication  of  cartilage,  the 
mesenchyme  in  these  regions  being-  converted  into  fibrous 
connective  tissue,  which,  especially  in  the  cranial  region, 
assumes  the  form  of  a  dense  membrane. 

But  in  addition  to  the  chondrocranium  and  the  vertebrae 
incorporated  with  it,  other  cartilaginous  elements  enter  into 
the  composition  of  the  skull.  The  mesenchyme  which 
occupies  the  axis  of  each  branchial  arch  undergoes  more  or 
less  complete  chondrification,  cartilaginous  bars  being  so 
formed,  certain  of  which  enter  into  very  close  relations 
with  the  skull.  It  has  been  seen  (p.  82)  that  each  half  of 
the  first  arch  gives  rise  to  a  maxillary  process  which  grows 
forward  and  ventrally  to  form  the  anterior  boundary  of  the 
mouth,  while  the  remaining  portion  of  the  arch  forms  the 
mandibular  process.  The  whole  of  the  axis  of  the  man- 
dibular process  becomes  chondrified,  forming  a  rod  known 
as  Meckel's  cartilage,  and  this,  at  its  dorsal  end,  comes  into 
relation  with  the  periotic  capsule,  as  does  also  the  dorsal 
end  of  the  cartilage  of  the  second  arch.  In  the  remaining 
three  arches  cartilage  forms  only  in  the  ventral  portions,  so 
that  their  rods  do  not  come  into  relation  with  the  skull, 
though  it  will  be  convenient  to  consider  their  further  his- 
tory together  with  that  of  the  other  branchial  arch  carti- 
lages. The  arrangement  of  these  cartilages  is  shown  dia- 
grammatically  in  Fig.  102. 

By  the  ossification  of  these  various  parts  three  categories 
of  bones  are  formed :  ( i )  cartilage  bones  formed  in  the 
chondrocranium,   (2)   membrane  bones,  and   (3)   cartilage 


OSSIFICATION    OF    THE    CHONDROCRANIUM. 


i8i 


bones  developing  from  the  cartilages  of  the  branchial  arches. 
The  bones  belonging  to  each  of  these  categories  are  pri- 
marily quite  distinct  from  one  another  and  from  those  of 
the  other  groups,  but  in  the  human  skull  a  very  considerable 
amount  of  fusion  of  the  primary  bones  takes  place,  and  ele- 
ments belonging  to  two  or  even  to  all  three  categories  may 
unite  to  form  a  single  bone  of  the  adult  skull.  In  a  certain 
region  of  the  chondrocranium  also  and  in  one  of  the  bran- 
chial arches  the  original  cartilage  bone  becomes  ensheathed 
by  membrane  bone  and 
eventually  disappears 
completely,  so  that  the 
adult  bone,  although 
represented  by  a  car- 
tilage, is  really  a  mem- 
brane bone.  And,  in- 
deed,   this    process    has 

proceeded     so      far     in 

r    .-,       Fig.   102. — Diagram   showing  the  Five 
certam   portions    of   the         branchial  Cartilages,  /  to  V. 

branchial     arch     skeleton    P,    Internal    pterygoid    process    of    the 

that  thp  nrio-inal  rartilao-  sphenoid;  At,  atlas;  Ax,  epistropheus; 
tnat  tne  original  cartliag-        ^^   ^■^■^^.^  cervical  vertebra. 

inous  representatives  are 

no  longer  developed,  but  the  bones  are  deposited  directly 
in  connective  tissue.  These  various  modifications  interfere 
greatly  with  the  precise  application  to  the  human  skull  of 
the  classification  of  bones  into  the  three  categories  given 
above,  and  indeed  the  true  significance  of  certain  of  the 
skull  bones  can  only  be  perceived  by  comparative  studies. 
Nevertheless  it  seems  advisable  to  retain  the  classification, 
indicating,  where  necessary,  the  confusion  of  bones  of  the 
various  categories. 

The  Ossification  of  the  Chondrocranium. — The  ossifi- 
cation of  the  cartilage  of  the  occipital  region  results  in  the 
formation  of  four  distinct  bones  which  even  at  birth  are 


l82 


OSSIFICATION    OF    THE    CHONDROCRANIUM. 


separated  from  one  another  by  bands  of  cartilage.  The 
portion  of  cartilage  lying  in  front  of  the  foramen  magnum 
ossifies  to  form  a  hasioccipital  bone  (Fig.  103,  ho),  the  por- 
tions on  either  side  of  this  give  rise  to  the  two  exoccipitals 
(eo),  which  bear  the  condyles,  and  the  portion  above  the 
foramen  produces  a  supraoccipital  {so),  which  represents 

the  part  of  the  squamous 
portion  of  the  adult  bone 
lying  below  the  superior 
nuchal  line.  All  that  por- 
tion of  the  bone  which 
lies  above  that  line  is  com- 
posed of  membrane  bone 
which  owes  its  origin  to 
the  fusion  of  two  or  some- 
times four  centers  of  ossi- 
fication, appearing  in  the 
membranous  roof  of  the 
embryonic  skull.  The  bone 
so  formed  {ip)  represents 
the  interparietal  of  lower 
vertebrates  and,  at  an  early 
stage,  unites  with  the  su- 


FlG. 


103. — Occipital     Bone 
Fetus  at  Term. 


bo,  Basioccipital;  eo,  exoccipital;  ip,  praoccipital,  although  even 
interparietal;   so,   supraoccipital.  ,  .     ,  •     ,■       •  r    ^ 

at  birth  an  nidication  ot  the 

line  of  union  of  the  two  parts  is  to  be  seen  in  two  deep  in- 
cisions at  the  sides  of  the  bone.  The  union  of  the  exoc- 
cipitals and  supraoccipital  takes  place  in  the  course  of  the 
first  or  second  year  after  birth,  but  the  basioccipital  does 
not  fuse  with  the  rest  of  the  bone  until  the  sixth  or  eighth 
year.  It  will  be  noticed  that  no  special  centers  occur  for 
the  four  occipital  vertebrae,  these  structures  having  become 
completely  incorporated  in  the  chondrocranium,  and  even  the 
cartilaginous  partitions  which  divide  the  hypoglossal  foram- 
ina usually  disappear  during  the  process  of  ossification. 


OSSIFICATION    OF    THE    CHONDROCRANIUM.  1 83 

Two  pairs  of  centers  have  been  described  for  the  inter- 
parietal bone  and  it  has  been  claimed  that  the  deep  lateral 
incisions  divide  the  lower  pair,  so  that  when  the  incisions 
meet  and  persist  as  the  sutura  mendosa,  separating  the  so- 
called  inca  bone  from  the  rest  of  the  occipital,  the  division 
does  not  correspond  to  the  line  between  the  supraoccipital 
and  the  interparietal,  but  a  portion  of  the  latter  bone  remains 
in  connection  with  the  supraoccipital.  Mall,  however,  in 
twenty  preparations,  found  but  a  single  pair  of  centers  for 
the  interparietal. 

Occasionally  an  additional  pair  of  small  centers  appear 
for  the  uppermost  angle  of  the  interparietal,  and  the  bones 


Fig.   104. — Sphenoid  Bone  from   Embryo  of  32  to  4  Months. 
The  parts  which  ai"e   still  cartilaginous   are   represented   in  black,     as, 
Alisphenoid ;    b,   basisphenoid ;    I,    lingula ;    os,    orbitosphenoid ;    p, 
internal  pterygoid  plate. —  (Sappey.) 

formed  from  them  may  remain  distinct  as  what  have  been 
termed  fontanelle  bones. 

In  the  sphenoidal  region  the  number  of  distinct  bones 
which  develop  is  much  greater  than  in  the  occipital  region. 
In  the  first  place,  at  the  beginning  of  the  ninth  week  a  center 
appears  in  each  of  the  cartilages  which  represent  the  alisphe- 
noids  (great  wings)  (Fig.  104,  as),  and  at  about  the  twelfth 
week  a  center  appears  in  each  orbitosphenoid  (lesser  wing) 
cartilage  (os).  A  little  later  a  pair  of  centers  (bs),  placed 
side  by  side,  are  developed  in  the  cartilage  representing  the 
posterior  portion  of  the  body  and  together  represent  what 


184  OSSIFICATION    OF    THE    CHONDROCRANIUM. 

is  known  as  the  basisphenoid,  and  still  later  a  center  appears 
on  either  side  of  the  basisphenoids  to  form  the  lingulce  (/), 
and  another  pair  appears  in  the  anterior  part  of  the  carti- 
lage, between  the  orbitosphenoids,  and  represent  the  pre- 
sphcnoid. 

In  addition  to  these  ten  centers,  all  of  which  are  formed 
in  cartilage,  certain  other  membrane  bones  are  included  in 
the  adult  sphenoid.  Thus,  a  little  before  the  appearance 
of  the  center  for  the  alisphenoids  an  ossification  is  formed 
in  the  mesenchyme  of  the  lateral  wall  of  the  posterior  part 
of  the  nasal  cavity  and  gives  rise  to  the  medial  lamina 
of  the  pterygoid  process,  the  mesenchyme  at  the  tip  of  the 
ossification  condensing  to  form  a  cartilaginous  hook-like 
structure  over  which  the  tendon  of  the  tensor  veli  palatini 
plays.  This  cartilage  later  ossifies  to  form  the  pterygoid 
hamulus,  the  medial  pterygoid  lamina  being  thus  a  combi- 
nation of  membrane  and  cartilage,  the  latter,  however,  being 
a  secondary  development  and  quite  independent  of  the  chon- 
drocranium.  It  is  probable  also  that  the  upper  anterior 
angle  of  each  alisphenoid  is  formed  by  ossification  of  the 
mesenchyme  in  this  situation  and  recent  observations  seem 
to  show  that  the  lateral  lamina  of  the  pterygoid  process 
develops  as  an  ossification  of  mesenchyme  situated  laterally 
to  the  medial  lamina  (Fawcett). 

The  lateral  pterygoid  laminae  early  unite  with  the  alisphe- 
noids, by  the  sixth  month  the  lingulae  have  fused  with  the 
basisphenoid  and  the  orbitosphenoids  with  the  presphenoid, 
and  a  little  later  the  basisphenoid  and  presphenoid  unite. 
The  alisphenoids  and  medial  pterygoid  laminae  remain  sepa- 
rate, however,  until  after  birth,  fusing  with  the  remaining 
portions  of  the  adult  bone  during  the  first  year. 

The  cartilage  of  the  ethmoidal  region  of  the  chondro- 
cranium  forms  somewhat  later  than  the  other  portions  and 
consists  at  first  of  a  stout  median  mass  projecting  down- 


OSSIFICATION    OF    THE    C H ON DRO CRANIUM. 


185 


ward  and  forward  into  the  nasal  process  (Fig.  105,  Ip), 
and  two  lateral  masses  (liu) ,  situated  one  on  either  side 
in  the  mesenchyme  on  the  outer  side  of  each  olfactory  pit. 
Ossification  of  the  lateral  masses  or  ectethnioids  begins  rela- 
tively early,  but  it  appears  in  the  upper  part  of  the  median 
cartilage  only  after  birth,  producing  the  crista  galH  and  the 
perpendicular  plate,  which  together  form  what  is  termed  the 
mesethnioid.  When  first  formed,  the  three  cartilages  are 
quite  separate  from  one  another,  the  olfactory  and  nasal 
nerves  passing-  down  between 
them  to  the  olfactory  pit,  but 
later  bony  trabeculse  begin  to 
extend  across  from  the  meseth- 
moid  to  the  upper  part  of  the 
ectethmoids  and  eventually  form 
a  fenestrated  horizontal  lamella 
which  ossifies  to  form  the  cribri- 
form plate. 

The  lower  part  of  the  median 
cartilage  does  not  ossify,  but  a 
center  appears  on  each  side  of  The  shaded  parts  represent  car- 


FiG.  105. — Anterior  Portion 
OF  THE  Base  of  the  Skull 
OF  A  6  TO  7  Months'  Em- 
bryo. 


tilage.  cp.  Cribriform  plate; 
Im,  lateral  mass  of  the  eth- 
moid ;  Ip,  perpendicular  plate ; 
of.  optic  foramen ;  os,  orbito- 
sphenoid. —  {After  von  Spec.) 


the  median  line  in  the  mesen- 
chyme behind  and  below  its  pos- 
terior or  lower  border.  From 
these  centers  two  vertical  bony 
plates  develop  which  unite  by  their  median  surfaces  below, 
and  above  invest  the  lower  border  of  the  cartilag"e  and  form 
the  vomer.  The  portion  of  the  cartilage  which  is  thus  in- 
vested undergoes  resorption,  but  the  more  anterior  portions 
persist  to  form  the  cartilaginous  septum  of  the  nose.  The 
vomer,  consequently,  is  not  really  a  portion  of  the  chondro- 
craniuin,  but  is  a  membrane  bone ;  its  intimate  relations  with 
the  median  ethmoidal  cartilage,  however,  make  it  conveni- 
ent to  consider  it  in  this  place. 
17 


1 86  OSSIFICATION    OF    THE    CHONDROCRANIUM. 

When  first  formed,  the  ectethmoids  are  masses  of  spongy 
bone  and  show  no  indication  of  the  honeycombed  appear- 
ance which  they  present  in  the  adult  skull.  This  condition 
is  produced  by  the  absorption  of  the  bone  of  each  mass  by 
evaginations  into  it  of  the  mucous  membrane  lining  the  nasal 
cavity.  This  same  process  also  brings  about  the  formation 
of  the  curved  plates  of  bone  which  project  from  the  inner 
surfaces  of  the  lateral  masses  and  are  known  as  the  superior 
and  middle  conchse  (turbinated  bones).  The  inferior  and 
sphenoidal  conchse  are  developed  from  special  centers,  but 
belong  to  the  same  category  as  the  others,  being  formed 
from  portions  of  the  lateral  ethmoidal  cartilages  which  be- 
come almost  separated  at  an  early  stage  before  the  ossifica- 
tion has  made  much  progress.  Absorption  of  the  body  of 
the  sphenoid  bone  to  form  the  sphenoidalcells,  of  the  frontal 
to  form  the  frontal  sinuses,  and  of  the  maxillaries  to  form 
the  maxillary  sinuses  is  also  produced  by  outgrowths  of  the 
nasal  mucous  membrane,  all  these  cavities,  as  well  as  the 
ethmoidal  cells,  being  continuous  with  the  nasal  cavities  and 
lined  with  an  epithelium  which  is  continuous  with  the  mucous 
membrane  of  the  nose. 

In  the  lower  mammalia  the  erosion  of  the  mesial  surface 
.  of  the  ectethmoidal  cartilages  results,  as  a  rule,  in  the  forma- 
tion of  five  conchae,  while  in  man  but  three  are  usually  recog- 
nized. Not  infrequently,  however,  the  human  middle  concha 
shows  indications,  more  or  less  marked,  of  a  division  into  an 
upper  and  a  lower  portion,  which  correspond  to  the  third  and 
fourth  bones  of  the  typical  mammalian  arrangement.  Further- 
more, at  the  upper  portion  of  the  nasal  wall,  in  front  of  the 
superior  concha,  a  slight  elevation,  termed  the  agger  nasi, 
is  always  observable,  its  lower  edge  being  prolonged  downward 
to  form  what  is  termed  the  uncinate  process  of  the  ethmoid. 
This  process  and  the  agger  together  represent  the  uppermost 
concha  of  the  typical  arrangement,  to  which,  therefore,  the 
human  arrangement  may  be  reduced. 

A  number  of  centers  of  ossification — the  exact  number 
is  yet  uncertain — appear  in  the  periotic  capsule  during  the 


OSSIFICATION    OF    THE    CHONDROCRANIUM.  1 87 

later  portions  of  the  fifth  month,  and  during  the  sixth  month 
these  unite  together  to  form  a  single  center  from  which  the 
complete  ossification  of  the  cartilage  proceeds  to  form  the 
petrous  and  mastoid  portions  of  the  temporal  bone  (Fig. 
106,  p).  The  mastoid  process  does  not  really  form  until 
several  years  after  birth,  being  produced  by  the  hollowing 
and  bulging  out  of  a  portion  of  the  petrous  bone  by  out- 
growths from  the  lining  membrane  of  the  middle  ear.  The 
cavities  so  formed  are  the  mastoid  cells,  and  their  relations 
to  the  middle-ear  cavity  are  in 
all  respects  similar  to  those  of 
the  ethmoidal  and  sphenoidal 
cells  to  the  nasal  cavities.  The 
remaining  portions  of  the  tem- 
poral bone  are  partly  formed  by 
membrane  bone  and  partly  from 
the  branchial  arch  skeleton. 
An    ossification   appears   at   the 

close  of  the  eighth  week  in  the 

,  ,  .  ,     .  .        .  ,      Fig.  106. — The  Temporal  Bone 

membrane  which  forms  the  side      ^^    Birth.      The    Styloid 

of    the    skull    in    the    temporal       Process  and  Auditory  Os- 

.  .  SICLES        ARE        NOT        RePRE- 

region     and    gives     rise    to    a      sented. 

squamosal  bone  (s),  which  later    P>  Petrous  bone;  ^squamosal; 
.  t,  tympanic. — {Poirier.) 

unites  with  the  petrous  to  form 

the  squamosal  portion  of  the  adult  temporal,  and  another 
membrane  bone,  the  tympanic  (t),  develops  from  a  center 
appearing  in  the  mesenchyme  surrounding  the  external 
auditory  meatus,  and  later  also  fuses  with  the  petrous  to 
form  the  floor  and  sides  of  the  external  meatus,  giving 
attachment  at  its  inner  edge  to  the  tympanic  membrane. 
Finally,  the  styloid  process  is  developed  from  the  upper  part 
of  the  second  branchial  arch,  whose  history  will  'be  consid- 
ered later. 

The  various  ossifications  which  form  in  the  chondrocra- 


OSSIFICATION    OF    THE    CHONDROCRANIUM. 


nium  and  the  portions  of  the  adult  skull  which  represent 
them  are  shown  in  the  following  table : 


Region  of 
Chonurocranium. 


Occipital, 


Sphenoidal, 


Ethmoidal,  . . . , 


Periotic  capsule. 


Ossification. 

Basioccipital 
Exoccipitals 
Supraoccipital 

'  Basisphenoid 
Presphenoid 
Lingiilae 
Alisphenoids 
Orbitosphenoids 

Mesethmoid 


Ectethmoids 

Inferior  concha. 
Sphenoidal  concha. 


Parts  of  Adult  Skull. 

Basilar  process. 
Condyles. 

Squamous  portion  below  su- 
perior nuchal  line. 

Body. 

Greater  wings. 
Lesser  wings. 
Lamina  perpendicularis. 
Crista  galli. 
Nasal  septum. 
Lateral  masses. 
Superior  concha. 
Middle  concha. 


Mastoid. 
Petrous. 


The  Membrane  Bones  of  the  Skull. — In  the  membrane 
forming  the  sides  and  roof  of  the  skull  in  the  second  stage 
of  its  development  ossifications  appear,  which  give  rise,  in 
addition  to  the  interparietal  and  squamosal  bones  already 
mentioned  in  connection  with  the  occipital  and  temporal,  to 
the  parietals  and  frontal.  Each  of  the  former  bones  de- 
velops from  a  single  center  which  appears  at  the  end  of 
the  eighth  week,  while  the  frontal  is  formed  at  about  the 
same  time  from  two  centers  situated  symmetrically  on  each 
side  of  the  median  line  and  eventually  fusing  completely 
to  form  a  single  bone,  although  more  or  less  distinct  indica- 
tions of  a  median  suture,  the  metopic,  are  not  infrequently 
present. 

Furthermore,  ossifications  appear  in  the  mesenchyme  of 
the  facial  region  to  iorm  the  nasaljachrymal,  and  zygomatic 
bones,  ah  'of  which  arise  from  single  centers  of  ossification. 
In  the  case  of  each  zygomatic  bone,  however,  three  osseous 
thickenings  appear  on  the  inner  surface  of  the  original  ossi- 


OSSIFICATION    OF    BRANCHIAL    ARCH    SKELETON.        1 89 

fication,  which  then  disappears  and  the  thickenings  unite  to 
form  the  adult  bone,  though  occasionally  one  or  more  of 
their  lines  of  union  may  persist,  producing  a  bipartite  or 
tripartite  zygomatic. 

The  vomer,  which  has  already  been  described,  belongs 
also  strictly  to  the  category  of  membrane  bones,  as  do  also 
the  maxilla  and  the  palatines ;  these  latter,  however,  pri- 
marily belonging  to  the  branchial  arch  skeleton,  with  which 
they  will  be  considered. 

The  purely  membrane  bones  in  the  skull,  are,  then,  the 
following : 

Interparietals,    Part  of  squamous  portion  of 

occipital. 

Squamosals, 0  Squamous  portions  of  tem- 
porals. 

Tympanies, Tympanic  plates  of  temporals. 

Parietals. 

Frontal. 

Nasals. 

Lachrymals. 

Zygomatics. 

Vomer. 

The  Ossification  of  the  Branchial  Arch  Skeleton. — It 

has  been  seen  (p.  i8o)  that  a  cartilaginous  bar  develops  only 
in  the  mandibular  process  of  the  first  branchial  arch.  In 
the  maxillary  process  no  cartilaginous  skeleton  forms,  but 
two  membrane  bones,  the  palatine  and  maxilla,  are  devel- 
oped in  it,  their  cartilaginous  representatives,  which  are  to 
be  found  in  lower  vertebrates,  having  been  suppressed  by 
a  condensation  of  the  development.  The  palatine  bone 
develops  from  a  single  center  of  ossification,  but  for  each 
maxilla  no  less  than  five  centers  have  been  described  (Fig. 
107).  One  of  these  gives  rise  to  so  much  of  the  alveolar 
border  of  the  bone  as  contains  the  bicuspid  and  molar  teeth ; 
a  second  forms  the  nasal  process  and  the  part  of  the  alveo- 
lar border  which  contains  the  canine  tooth ;  a  third  the  por- 
tion which  contains  the  incisor  teeth;  while  the  fourth  and 


IQO      OSSIFICATION    OF    BRANCHIAL    ARCH    SKELETON. 


fifth  centers  lie  above  the  first  and  give  rise  to  the  inner  and 
outer  portions  of  the  orbital  plate  and  the  body  of  the  bone. 
The  first,  second,  fourth,  and  fifth  portions  early  unite  to- 
gether, but  the  third  center,  which  really  lies  in  the  ven- 
tral part  of  the  nasal  process,  remains  separate  for  some 
time,  forming  what  is  termed  the  premaxilla,  a  bone  which 
remains  permanently  distinct  in  the  majority  of  the  lower 
mammals. 

The  above  is  the  generally  accepted  view  as  to  the  develop- 
ment of  the  maxilla.      Mall,  however,  maintains  that  it  has 

but  two  centers  of  ossification, 
one  giving  rise  to  the  pre- 
maxilla and  the  other  to  the 
rest  of  the  bone.  The  max- 
illary center  makes  its  appear- 
ance about  the  middle  of  the 
sixth  week. 

Since  the  condition  known 
as  hare-lip  results  from  a  fail- 
ure of  the  maxillary  process 
to  unite  completely  with  the 
frontonasal  process  (see  p. 
89),  and  since  the  premax- 
illa develops  in  the  latter  and 
the  maxilla  in  the  former,  the  cleft  may  pass  between  these 
two  bones  and  prevent  their  union  (see  also  p.  301). 

The  upper  end  of  Meckel's  cartilage  passes  between  the 
tympanic  bone  and  the  outer  surface  of  the  periotic  cap- 
sule and  thus  comes  to  lie  apparently  within  the  tympanic 
cavity  of  the  ear;  this  portion  of  the  cartilage  divides  into 
two  parts  which  ossify  to  form  two  of  the  bones  of  the  mid- 
dle ear,  the  malleus  and  incus,  a  description  of  whose 
further  development  may  be  postponed  until  a  later  chap- 
ter. At  about  the  middle  of  the  sixth  week  of  development 
a  plate  of  membrane  bone  appears  to  the  outer  side  of  the 
lower  portion  of  the  cartilage  and  gradually  extends  to  form 
the  greater  portion  of  the  body  of  the  mandible,  the  ramuis 


Fig.  107. — Diagram  of  the  Os- 
sifications OF  which  the  Max- 
illa is  Composed,  as  seen  from 
THE  Outer  Surface.  The 
Arrow  Passes  through  the 
Infraorbital  Canal. —  {From 
von  Spee,  after  Sappey.) 


OSSIFICATION    OF    BRANCHIAL    ARCH    SKELETON.        I9I 

and  the  coronoid  process.  In  the  region-  of  the  body  the 
bone  develops  so  as  to  enclose  the  cartilage,  together  with 
the  inferior  alveolar  (dental)  nerve  which  lies  to  the  outer 
side  of  the  cartilage,  but  in  the  region  of  the  ramus  the 
bone  remains  entirely  to  the  outer  side  of  the  cartilage  and 
nerve,  whence  the  position  of  the  mandibular  foramen  on 
the  inner  surface  of  the  adult  bone.  The  portion  of  Meck- 
el's cartilage  extending  from  the  symphysis  to  the  level  of 
the  mental  foramen  ossifies  to  form  the  mental  portion  of 
the  mandible,  but  throughout  the  rest  of  the  body  of  the 
bone  it  disappears,  while  the  portion  above  the  mandibular 
foramen  is  said  to  become  transformed  into  fibrous  connec- 
tive tissue  and  to  persist  as  the  spheno-mandibular  ligament. 
At  the  upper  extremity  of  the  bony  ramus  two  nodules  of 
cartilage  develop,  quite  independently,  however,  of  Meck- 
el's cartilage,  and  these  ossify  to  form  the  condyloid  and 
coronoid  processes,  so  that  each  half  of  the  mandible  is 
formed  of  membrane  bone,  with  cartilage  bone  at  each 
extremity. 

The  upper  part  of  the  cartilage  of  the  second  branchial 
arch  also  comes  into  relation  with  the  tympanic  cavity 
and  ossifies  to  form  the  styloid  process  of  the  temporal 
bone.  The  succeeding  moiety  of  the  cartilage  underg-oes 
degeneration  to  form  the  stylo-hyoid  ligament,  while  its 
most  ventral  portion  ossifies  as  the  lesser  cornii  of  the 
hyoid  hone.  The  great  variability  which  may  be  observed 
in  the  length  of  the  styloid  processes  and  of  the  lesser 
cornua  of  the  hyoid  depends  upon  the  extent  to  which  the 
ossification  of  the  original  cartilage  proceeds,  the  length 
of  the  stylo-hyoid  ligaments  being  in  inverse  ratio  to  the 
length  of  the  processes  or  cornua.  The  greater  cornua  of 
the  hyoid  are  formed  by  the  ossification  of  the  cartilages 
of  the  third  arch,  and  the  body  of  the  bone  is  formed  from 
a  cartilaginous  plate,  the  copula,  which  unites  the  ventral 
ends  of  the  two  arches  concerned. 


192      OSSIFICATION    OF    BRANCHIAL   ARCH    SKELETON. 


Finally,  the  cartilages  of  the  fourth  and  fifth  branchial 
arches  early  fuse  together  to  form  a  plate  of  cartilage,  and 
^;he  two  plates  of  opposite  sides  unite  by  their  ventral  edges 
to  form  the  thyreoid  cartilage  of  the  larynx. 


Fig.  108. — Diagram  showing  the  Categories  to  which  the  Bones 
OF  the  Skull  Belong. 

The  unshaded  bones  are  membrane  bones,  the  heavily  shaded  repre- 
sent the  chondrocranium,  while  the  black  represents  the  branchial 
arch  elements.  AS,  Alisphenoid ;  ExO,  exoccipital;  F,  frontal; 
Hy,  hyoid ;  IP,  interparietal ;  Z,  zygomatic ;  Mn,  mandible ;  Mx, 
maxilla;  NA,  nasal;  P,  parietal;  Pe,  periotic;  SO,  supraoccipital ; 
Sq,  squamosal;  St,  styloid  process;  Th,  thyreoid  cartilage;  Ty, 
tympanic. 

The  accompanying  diagram   (Fig.  108)  shows  the  vari- 
ous structures  derived  from  the  branchial  arch  skeleton,  as 


DEVELOPMENT    OF    APPENDICULAR    SKELETON.  1 93 

well  as  some  of  the  other  elements  of  the  skull,  and  a 
resume  of  the  fate  of  the  hranchial  arches  may  be  stated  in 
tabular  form  as  follows,  the  parts  represented  by  cartilage 
which  becomes  replaced  by  membrane  bone  being-  printed  in 
italics,  while  membrane  bones  which  have  no  cartilaginous 
representatives  are  enclosed  in  brackets : 

f  (Maxilla). 
(Palatine). 
Malleus. 
Incus. 

Spheno-mandibular    ligament. 
Mandible. 


1st   arch, 


f-  Styloid    process     of    the     temporal. 

2d    arch,    -|    Stylo-hyoid  ligament. 

(^  Lesser  cornu  of  hyoid. 

3d    arch, Greater  cornu  of  hyoid. 

4th  and  5th  arches, ....       Thyreoid  cartilage  of  larynx. 

The   Development  of  the  Appendicular   Skeleton. — 

While  the  greater  portion  of  the  axial  skeleton  is  formed 
from  the  sclerotomes  of  the  mesodermic  somites,  the  ap- 
pendicular skeleton  is  derived  from  the  somatic  mesen- 
chyme, which  is  not  divided  into  metameres.  This  mesen- 
chyme forms  the  core  of  the  limb  bud  and  becomes  con- 
verted into  cartilage,  by  the  ossification  of  which  all  the 
bones  of  the  limbs,  with  the  possible  exception  of  the  clavi- 
cle, are  formed. 

Of  the  bones  of  the  pectoral  girdle  the  clavicle  requires 
further  study  before  it  can  be  certain  whether  it  is  to  be 
regarded  as  a  purely  cartilage  bone  or  as  a  combination  of 
cartilage  and  membrane  ossifications  (Gegenbaur).  It  is 
the  first  bone  of  the  skeleton  to  ossify,  two  centers  appear- 
ing for  each  bone  at  about  the  sixth  week  of  development. 
The  tissue  in  which  the  ossifications  form  has  certain  pecu- 
liar characters,  and  it  is  difficult  to  say  whether  it  is  to  be 
18 


194    DEVELOPMENT  OF  APPENDICULAR  SKELETON. 


regarded  as  cartilage  which,  on  account  of  the  early  differ- 
entiation of  the  center,  has  not  yet  become  thoroughly 
differentiated  histologically,  or  as  some  other  form  of  con- 
nective tissue.  However  that  may  be,  true  cartilage  devel- 
ops on  either  side  of  the  ossifying  region,  and  into  this  the 
ossification  gradually  extends,  so  that  at  least  a  portion  of 
the  bone  is  preformed  in  cartilage. 

The  scapula  is  at  first  a  single  plate  of  cartilage  in  which 

two  centers  of  ossification 
appear.  One  of  these 
gives  rise  to  the  body  and 
the  spine,  while  the  other 
produces  the  coracoid  proc- 
ess (Fig.  109,  co),  the 
rudimentary  representative 
of  the  coracoid  bone  which 
extends  between  the  scap- 
ula and  sternum  in  the 
lower  vertebrates.  The 
coracoid  does  not  unite 
with  the  body  until  about 
the  fifteenth  year,  and  sec- 
FiG.    109. — The    Ossification    Cen-   ondary  centers  which  give 

TERS    OF   THE    ScAPULA.  •  i.         i-1  i    1         1  1 

rise  to  the   vertebral  edge 

a,    b,    and    c.    Secondary    centers    for  .  '^ 

the    angle,    vertebral    border,    and     (h)    and    inferior   angle    of 
acromion;   co,  center  for  the  cora-    ,i        u^,,^     /„\     „,^j    +^    iU„ 

coid  pvoct,s.-{Tcstut.)  the  bone    (a)    and   to   the 

acromion  process  (c)  unite 
with  the  rest  of  the  bone  at  about  the  twentieth  year. 

The  huincnis  and  the  bones  of  the  forearm  are  typical 
long  bones,  each  of  which  develops  from  a  primary  center, 
which  gives  rise  to  the  shaft,  and  has,  in  addition,  two  or 
more  epiphysial  centers.  In  the  humerus  an  epiphysial 
center  appears  for  the  head,  another  for  the  greater  tuber- 
osity, and  usually  a  third  for  the  lesser  tuberosity,  while 
at  the  distal  end  there  is  a  center  for  each  condyle,  one  for 


DEVKLOPMENT    OF    APPENDICULAR    SKELETON.  1 95 

the  trochlea  and  one  for  the  capitnlum.  the  fusion  of  these 
various  epiphyses  with  the  shaft  taking  place  between  the 
seventeenth  and  twentieth  years.  The  radius  and  ulna  each 
possess  a  single  epiphysial  center  for  each  extremity  in  addi- 
tion to  the  primary  center  for  the  shaft,  and  the  ulna  pos- 
sesses also  an  epiphysial  center  for  the  olecranon  process. 

The  embryological  development  of  the  carpus  is  some- 
what complicated.     A  cartilage  is  found  representing  each 
of  the  bones  normally  occurring  in  the  adult   (Fig.   no), 
and  these  are  arranged 
in  two  distinct  rows :  a 
proximal  one  consisting 
of  three  elements,  named 
from    their    relation    to 
the   bones    of   the    fore- 
arm,    radialc,     intcrinc- 

diuin,  and  ulnar c ;  and  a         '  r^W       \  "  l 

distal  one  composed  of 
four  elements,  termed 
carpalia.  In  addition,  a 
cartilage,      termed      the  .    , 

pisiform,  is  found  on  the    Fig.    no.— Reconstruction   of  an   Em- 

ulnar  side  of  the  proxi-  ^"^^^^^  ^^^^^'- 

r,  Centrale;  cu,  triquetral;  In,  lunate; 
mal  row  and  is  generally  „;^  capitate;  f-  pisiform;  sc,  navicular; 
regarded    as    a    sesamoid        f'  g^-^fter  multangular;  tr,  lesser  mul- 

°  .  ,  tangular;  ti,  hamate, 

cartilage  developed  in  the 

tendon  of  the  flexor  carpi  ulnaris,  and  furthermore  a  number 
of  inconstant  cartilages  have  been  observed  whose  signifi- 
cance in  the  majority  of  cases  is  more  or  less  obscure. 
These  accessory  cartilages  either  disappear  in  later  stages 
of  development  or  fuse  with  neighboring  cartilages,  or,  in 
rare  cases,  ossify  and  form  distinct  elements  of  the  carpus. 
One  of  them,  howcA-er,  occurs  so  frequently  as  almost  to 
deserve  classification  as  a  constant  element ;  it  is  known  as 


196  DEVELOPMENT    OF    APPENDICULAR    SKELETON. 

the  ccntralc  (Fig.  no,  c)  and  occupies  a  position  between 
the  cartilages  of  the  proximal  and  distal  rows  and  appa- 
rently corresponds  to  a  cartilage  typically  present  in  lower 
forms  and  ossifying  to  form  a  distinct  bone.  In  the  human 
carpus  its  fate  varies,  as  it  may  either  disappear  or  unite 
with  other  cartilages,  that  with  which  it  most  usually  fuses 
being  probably  the  radiale.  There  is  evidence  also  to  show 
that  another  of  the  accessory  cartilages  unites  with  the 
ulnar  element  of  the  distal  row,  representing  the  carpale  v 
typically  present  in  lower  forms. 

Each  of  the  elements  corresponding  to  an  adult  bone 
ossifies  from  a  single  center  with  the  exception  of  carpale 
iV-v  which  has  two  centers,  a  further  indication  of  its 
composite  character.  The  relation  of  the  cartilages  to  the 
adult  bones  may  be  seen  from  the  table  given  on  page  198. 

With  regard  to  the  metacarpals  and  phalanges,  it  need 
merely  be  stated  that  each  develops  from  a  single  primary 
center  for  the  shaft  and  one  secondary  epiphysial  center. 
The  primary  center  appears  at  about  the  middle  of  the  shaft 
except  in  the  terminal  phalanges,  in  which  it  appears  at  the 
distal  end  of  the  cartilage.  The  epiphyses  for  the  meta- 
carpals are  at  the  distal  ends  of  the  bones,  except  in  the 
case  of  the  metacarpal  of  the  thumb,  which  resembles  the 
phalanges  in  having  its  epiphysis  at  the  proximal  end. 

Each  iiuiomiiiafe  bone  appears  as  a  somewhat  oval  plate 
of  cartilage  whose  long  axis  is  directed  almost  at  right 
angles  to  the  vertebral  column  and  which  is  in  close  rela- 
tion with  the  fourth  and  fifth  sacral  vertebrae.  As  devel- 
opment proceeds  a  rotation  of  the  cartilage,  accompanied 
by  a  slight  shifting  of  position,  occurs,  so  that  eventually 
the  plate  has  its  long  axis  almost  parallel  with  the  vertebral 
column  and  is  in  relation  with  the  first  three  sacrals.  Ossi- 
fication appears  at  three  points  in  each  cartilag'e,  one  in  the 
upper  part  to  form  the  ilium   (Fig.  in,  il)  and  two  in  the 


DEVELOPMENT    OF    APPENDICULAR    SKELETON. 


197 


lower  part,  the  anterior  of  these  giving  rise  to  the  pubis 
(p),  while  the  posterior  produces  the  ischium  (is).  At 
birth  these  three  bones  are  still  separated  from  one  another 
by  a  Y-shaped  piece  of  cartilage  whose  three  limbs  meet  at 
the  bottom  of  the  acetabulum,  but  later  a  secondary  center 
appears  in  this  cartilage  and  unites  the  three  bones  together. 
The  central  part  of  the 
lower  half  of  each  original 
cartilage  plate  does  not 
undergo  complete  chondri- 
fication,  but  remains  mem- 
branous, constituting  the 
obturator  membrane  which 
closes  the  obturator  fora- 
men. 

In  addition  to  the  Y- 
shaped  secondary  center, 
other  epiphysial  centers 
appear  in  the  prominent 
portions    of    the    cartilage, 

such    as    the    pubic    crest  _      ^  r^ 

.  Fig.  III.— The  Ossification  Centers 

(Fig.    Ill,    c),   the   ischial  of  the  Os  Innominatum. 

tuberosity  (d),  the  anterior    a,  b,  c,  and  d,  Secondary  centers  for 
...  .         /  7  N  11  the    crest,    anterior    nifenor    spine, 

inferior  Spme    {b)    and  the        svmphvsis,    and   ischial   tuberosity; 

crest  of  the  ilium  (o),  and       ^I'JT^I  '''  ischium;  p,  pubis.- 

^    ^\  {Test  lit.) 

unite  with  the  rest  of  the 

bone  at  about  the  twentieth  year. 

The  femur,  tibia,  and  fibula  each  develop  from  a  single 
primary  center  for  the  shaft  and  an  upper  and  a  lower 
epiphysial  center,  the  femur  possessing,  in  addition,  epi- 
physial centers  for  the  greater  and  lesser  trochanters  (Fig. 
91).  The  patella  does  not  belong  to  the  same  category 
as  the  other  bones,  but  resembles  the  pisiform  bone  of  the 
carpus  in  being  a  sesamoid  bone,  developed  in  the  tendon 


198 


DEVELOPMENT    OF    APPENDICULAR    SKELETON. 


of  the  quadriceps  extensor  cruris.  Its  cartilage  does  not 
appear  until  the  fourth  month  of  intrauterine  life,  when 
most  of  the  primary  centers  for  other  bones  have  already 
appeared,  and  its  ossification  does  not  begin  until  the  third 
year  after  birth. 

The  torsiis,  like  the  carpus,  consists  of  a  proximal  row 
of  three  cartilages,  termed  the  tihialc,  the  intcrincdium,  and 
the  fihularc,  and  of  a  distal  row  of  four  tarsalia.  Between 
these  two  rows  a  single  cartilage,  the  ccntrah\  is  interposed. 
Each  of  these  cartilages  ossifies  from  a  single  center,  that 
of  the  intermedium  early  fusing  with  the  tibiale,  though  it 
occasionally  remains  distinct  as  the  os  trigomini,  and  from 
a  comparison  with  lower  forms  it  seems  probable  that  the 
fibular  cartilage  of  the  distal  row  really  represents  two 
separate  elements,  there  being,  properly  speaking,  five  tar- 
salia instead  of  four.  The  fibulare,  in  addition  to  its  pri- 
mary center,  possesses  also  an  epiphysial  center,  which 
develops  at  the  point  of  insertion  of  the  tendo  Achillis. 

A  comparison  of  the  carpal  and  tarsal  cartilages  and 
their  relations  to  the  adult  bones  may  be  seen  from  the  fol- 
lowinar  table : 


Carpus. 

Tarsus. 

Cartilages. 

Bones 

Bones 

Cartilages. 

Radiale 

Navicular 

Talus 

f  Tibiale 

Intermedium 

Lunate 

\  Intermedium 

Ulnare 

Triquetral 

Calcaneus 

Fibulare 

Sesamoid  cartilage 

Pisiform 

Centrale 

Pauses  with  navicu- 

Navicular 

Centrale 

Carpale      I 

Gr.  multangular 

1st  Cuneiform 

Tarsal  e      I 

Carpale     II 

Less,  multangular 

2d  Cuneiform 

Tarsal  e    11 

Carpale  III 

Capitate 

3d  Cuneiform 

Tarsal e  III 

Carpale    IV  ) 
Carpale     V   J 

Hamate 

Cuboid 

r  Tarsale   IV 
\  Tarsale     V 

The  develojiment  of  the  metatarsals  and  phalanges  is 
exactly  similar  to  that  of  the  corresponding  bones  of  the 
hand  (see  p.  196). 


DEVELOPMENT    OF    THE    JOINTS.  1 99 

The  Development  of  the  Joints. — The  mesenchyme 
which  primarily  represents  each  vertebra,  or  the  skull,  or 
the  skeleton  of  a  limb,  is  at  first  a  continuous  mass,  and 
when  it  becomes  converted  into  cartilage  this  also  may  be 
continuous,  as  in  the  skull,  or  may  appear  as  a  number  of 
distinct  parts  united  by  unmodified  portions  of  the  mesen- 
chyme. In  the  former  case  the  various  ossifications  as  they 
extend  will  come  into  contact  with  their  neighbors  and  will 
either  fuse  with  them  or  will  articulate  with  them  directly, 
forming"  a  suture. 

When,  ho^^'ever,  a  portion  of  unmodified  mesenchyme 
intervenes  between  two  cartilages,  the  mode  of  articulation 
of  the  bones  formed  from  these  cartilages  will  vary.  The 
intermediate  mesenchyme  may  in  time  undergo  chondrifi- 
cation  and  unite  the  bones  in  an  almost  immovable  articu- 
lation known  as  a  synchondrosis  {e.  g.,  the  sacroiliac  articu- 
lation) ;  or  a  cavity  may  appear  in  the  center  of  the  inter- 
vening cartilage  so  that  a  slight  amount  of  movement  of 
the  two  bones  is  possible,  forming"  an  ainphiarthrosis  {e.  g., 
the  symphysis  pubis)  ;  or,  finally,  the  intermediate  mesen- 
chyme may  not  chondrify,  but  its  peripheral  portions  may 
become  converted  into  a  dense  sheath  of  connective  tissue 
(Fig.  112,  c)  which  surrounds  the  adjacent  ends  of  the 
two  bones  like  a  sleeve,  forming  the  articular  capsule,  while 
the  central  portions  degenerate  to  form  a  cavity.  The  bones 
which  enter  into  such  an  articulation  are  more  or  less  freely 
movable  upon  one  another  and  the  joint  is  termed  a  diar- 
throsis  (e.  g.,  the  knee-  or  shoulder-joint). 

In  a  diarthrosis  the  connective-tissue  cells  near  the  inner 
surface  of  the  capsule  arrange  themselves  in  a  layer  to  form 
a  synovial  membrane  for  the  joint,  and  portions  of  the  cap- 
sule may  thicken  to  form  special  bands,  the  reinforcing 
ligaments,  while  other  strong  fibrous  bands,  which  may  pass 
from  one  bone  to  the  other,  forming  accessory  ligaments, 


200  DEVELOPMENT    OF    THE    JOINTS. 

are  shown  by  comparative  studies  to  be  in  many  cases  de- 
generated portions  of  what  were  originally  muscles. 

In  certain  diarthroses.  such  as  the  temporo-mandibular 
and  sterno-clavicular,  the  whole  of  the  central  portions  of 
the  intermediate  mesenchyme  does  not  degenerate,  but  it 
is  converted  into  a  fibro-cartilage,  between  each  surface  of 
which  and  the  adjacent  Ixme  there  is  a  cavity.     These  inter- 


c—' 


Fig.   112. — Longitudinal  Section  through  the  Joint  of  the  Great 

Toe  in  an  Embryo  of  4.5  cm. 
c,   Articular    capsule ;    /,    intermediate    mesenchyme    which    has    almost 

disappeared   in   the   center ;    f^^   and   p',   cartilages   of   the   first    and 

second  phalanges. —  {Nicholas. ) 

articular  cartilages  seem,  in  the  sterno-clavicular  joints,  to 
represent  the  sternal  ends  of  a  bone  existing  in  lower  verte- 
brates and  known  as  the  prccoracoid ,  but  it  seems  doubtful 
if  those  of  the  temporo-mandibular  joints  have  a  similar 
significance,  the  most  recent  observations  on  their  develop- 
ment tending  to  associate  them  with  the  external  pterygoid 
muscles  (Kjellberg). 

LITERATURE. 

C.  R.  Bardeen  :  "  The  Development  of  the  Thoracic  Vertebrae  in  Man." 

Amcr.  Journ.  Anat.,  iv,   1905. 
C.  R.  Bakdeen  :  "  Studies  of  the  Development  of  the  Human  Skeleton,'' 

Amer.  Journ.  Anat.,  iw,  1905. 


LITERATURE.  201 

'  A.  Bernays  :  "  Die  Entwicklungsgeschichte  des  Kniegelenks  des  Men- 

schen  mit  Bemerkungen  iiber  die  Gelenke  im  AllfTemeinen,"  Mor- 
pholog.   Jahrbuch,  iv,   1878. 
E.    Dursy:    "  Zur    Entwicklungsgeschichte    des    Kopfes    des    Menschen 
/  und   der  hoheren  Wirbelthiere,"  Tiibingen,   1869. 

V  E.  Fawcett  :  "  On  the  early  Stages  in  the  Ossification  of  the  Pterygoid 

Plates  of  the  Sphenoid  Bone  of  Man,"  Aiiaf.  Anseiger,  xxvi,  1905. 
W        E.  Fawcett:  "Ossification  of  the  lower  Jaw  in  Man,"  Joiirn.  Amcr. 
Med.  Assoc,  XLV,  1905. 

E.  Fawcett  :    "  On   the   development,   ossification   and   growth    of   the 

Palate  bone,"  Journ.  Anat.  and  Phys.,  xl^  1906. 
A.   Froriep  :    "  Zur   Entwicklungsgeschichte   der  Wirbelsaulc,   insbeson- 
dere  des  Atlas  und  Epistropheus  und  der  Occipitalregion,"  Archiv 
fiir  Anat.  und  Physiol.,  Anat.  Abih.,  1886. 

V  E.  Gaupp  :  "  Alte  Probleme  und  neuere  Arbeiten  iiber  den  Wirbeltier- 

schadel,"   Ergeb.    der  Anat.    und  Enttvieklungsgesch.,  x,    1901. 
C.  Gegenbaur  :   "  Fin  Fall  von  erblichem  Mangel  der  Pars  acromialis 
Claviculje,  mit  Bemerkungen  iiber  die  Entwicklung  der  Clavicula," 
Jenaische  Zeitsclir.  fiir  medic.   Wissensch.,  i,  1864. 

V  E.  Grafenberg  :  "  Die  Entwicklung  der  Knochen,  Muskeln  und  Nerven 

der    Hand    und    der    fiir    die    Bewegungen    der    Hand    bestimmten 

Muskeln   des   Unterarms,"   Anat.  Hefte,   xxx,   1906. 
Henke  and  Reyher :   "  Studien  iiber  die  Entwickelung  der  Extremi- 

taten    des    Menschen,    insbesondere    der    Gelenkflachen,"    Sitcungs- 

berichte  der  kais.  Akad.  Wien,  lxx,  1875. 
M.    Jakoby  :    "  Beitrag    zur    Kenntnis    des    menschlichen    Primordial- 

craniums,"  Archiv  fiir  mikrosk.  Anat.,  xliv^  1894. 
'y.  K.     KjELLBERG :     "  Bcitrage     zur    Entwicklungsgeschichte    des     Kiefer- 

gelenks,"  Morph.  Jahrbuch,   xxxii,   1904. 
H.  Leboucq  :  "  Recherches  sur  la  morphologic  du  carpe  chez  les  mam- 

miferes,"  Archives  de  Biolog.,  v,  1884. 
G.    Levi  :    "  Beitrag   zum    Studium   der   Entwickelung   des   knorpeligen 

Primordialcraniums    des    Menschen,"    Archiv    fiir    inikroslc.    Anat., 

LV,  1900. 
A.  Low  :  "  The  Development  of  the  Lower  Jaw  in  Man,"  Proc.  Anat.  and 

Antlirop.  Soc.  Univ.  of  Aberdeen,  1906. 
i^p    P.  Mall:  "The  Development  of  the  Connective  Tissues  from  the 

Connective-tissue   Syncytium,"   Amer.  Jour.  Anat.,  i,   1902. 

F.  P.  Mall  :  "  On  Ossification  Centers  in  Human  Embryos  Less  Than 

V 

One  Hundred  Days  Old,"  Amcr.  Journ.  Anat.,  v,  1906. 
J.    Markowski:    "  Sollte    der   Verknocherungsprozess     des     Brustbeins 
von    keiner    morphologischen    Bedeutung   sein?"     Anat.    Anzcigcr, 
XXVI,   1905. 


202  LITERATURE. 

W.   VAN   NooRDEN :    "  Beitrag   zur   Anatomic   der   knorpeligen   Schadel- 

basis   menschlicher    Embryonen,"   Archiv   fiir  Anat.    tind   Physiol., 

Anat.  Abth.,  1887. 
A.  M.  Paterson  :  "  The  Human  Sternum,"  Liverpool,  1904. 
K.  Peter  :  "  Anlage  und  Homologie  der  Muscheln  des  Menschen  und 

der  Saugetiere,"  Arch,  fiir  mikrosk.  Anat.,  Lx,  1902. 
Rambaut   et   Renault  :    "  Origine   et   developpement   des    Os,"    Paris, 

1864. 
E.    Rosenberg  :    "  Ueber   die    Entwickelung   der    Wirbelsiiule    und    das 

Centrale  carpi  des  Menschen,"  Morpholog.  Jahrbuch,  i,  1876. 
G.  Ruge:  "  Untersuchungen  iiber  die  Entwickelungsvorgange  am  Brust- 

bein  des  Menschen,"  Morpholog.  Jahrbuch,  vi,  1880. 
G.  Thilenius  :  "  Untersuchungen  iiber  die  morphologische  Bedeutung 

accessorischer  Elemente  am  menschhchen   Carpus    (und  Tarsus)," 

Morpholog.  Arbeitcn,  v,  1896. 
K.    ToLDT   Jr.  :    "  Entwickhmg   und    Struktur   des    menschlichcn    Joch- 

beines,"   Sitsungsber.   k.   Acad.    Wisscnsch.   Wien,  Math.-naturzviss 

KL,  CXI,  1902. 
A.    Weiss  :    "  Die    Entwicklung    der    Wirbelsiiule    der    weissen    Ratte, 

besonders    der   vordersten    Halswirbel,"   Zeit.    f.    luissensch.    Zool., 

Lxix,   1 901. 
P.  A.  Zachariades  :   "  Recherches  sur  le  developpement  du  tissu  con- 

jonctif,"   Comptes  Rcndiis  dc    la  Soc.    dc   Biolog.,   Paris,    Sen    10, 

v,    i8q8. 


CHAPTER    VIII. 

THE  DEVELOPMENT  OF  THE  MUSCULAR 
SYSTEM. 

Two  forms  of  muscular  tissue  exist  in  the  human  body, 
the  striated  tissue,  which  forms  the  skeletal  muscles  and  is 
under  the  control  of  the  central  nervous  system,  and  the 
non-striated,  which  is  controlled  by  the  sympathetic  ner- 
vous system  and  is  found  in  the  skin,  in  the  walls  of  the 
digestive  tract,  the  blood-vessels  and  lymphatics,  and  in 
connection  with  the  genito-urinary  apparatus.  In  the  walls 
of  the  heart  a  muscle  tissue  occurs  which  is  frequently  re- 
garded as  a  third  form,  characterized  by  being  under  con- 
trol of  the  sympathetic  system  and  yet  being  striated;  it 
is,  however,  in  its  origin,  much  more  nearly  allied  to  the 
non-striated  than  to  the  striated  form  of  tissue,  and  will 
be  considered  a  variety  of  the  former. 

The  Histogenesis  of  Non-striated  Muscular  Tissue. — 
Non-striated  muscular  tissue  is  formed  by  the  direct  con- 
version of  mesenchyme  cells  into  muscle-fibers,  the  exact 
details  of  the  conversion  being  as  yet  unknown.  The 
fibers  are  sometimes  more  or  less  scattered  in  the  general 
connective  tissue  or  may  be  grouped  into  small  bundles  or 
into  layers.  They  are  formed  from  the  mesenchyme  of  the 
somatic  and  splanchnic  layers  of  the  mesoderm,  but  appa- 
rently never  from  the  mesodermic  somites. 

The  cells  from  which  the  heart  musculature  develops  are 
at  first  of  the  usual  well  defined  embryonic  type,  but,  as 
development  proceeds,  they  become  irregularly  stellate  in 
form,   the  processes   of  neig"hboring  cells   fuse  and,  even- 

203 


204 


DEVELOPMENT    OF    MUSCULAR    SYSTEM. 


tually,  there  is  formed  a  continuous  mass  of  protoplasm  or 
syncytium  in  which  all  traces  of  cell  boundaries  are  lackinsr 
(Fig.    113).     While  the  individual   cells,  or  myoblasts  as 

they  are  termed,  are 
still  recognizable,  gran- 
ules appear  in  their  cyto- 
plasm, and  these  arrange 
themselves  in  rows  and 
unite  to  form  slender 
fibrils,  W'hich  at  first 
do  not  extend  beyond 
the  limits  of  the  myo- 
blasts in  which  they 
have  appeared,  but  later, 
as  the  fusion  of  the 
cells  proceeds,  are  con- 
tinued from  one  cell 
territory  into  the  other 
through         considerable 

Fig.  113. — Section  through  the  Heart-    stretches    of    the    syncy- 
WALL  OF  A  Duck  Embryo  of  Three      .  -.i        .  1    - 

Days.-(M.  Hcidcnhain.)  tium,  Without  regard  to 

the  original  cell  areas. 
The  fibrils  multiply,  apparently  by  longitudinal  division, 
and  arrange  themselves  in  circles  around  areas  of  the  syn- 
cytium (compare  Fig.  114).  As  the  multiplication  of  the 
fibrils  continues  those  newly  formed  arrange  themselves 
anjund  the  interior  of  each  of  the  original  circles  and  grad- 
ually occupy  the  entire  cytoplasm,  or  sarcoplasm  as  it  may 
now  be  termed,  except  immediately  around  the  nuclei 
where,  even  in  the  adult,  a  certain  amount  of  undifferen- 
tiated sarcoi)lasm  persists.  The  fibrils  when  first  formed 
are  apparently  homogeneous,  but  later  they  become  differ- 
entiated into  two  distinct  substances  which  alternate  with 


HISTOGENESIS    OF    STRIATED    MUSCLE    TISSUE. 


205 


one  another  throughout  the  length  of  the.  fibril  and  produce 
the  characteristic  transverse  striation  of  the  tissue.  Finally 
stronger  interrupted  transverse  bands  of  so-called  cement 
substance  appear,  dividing  the  tissue  into  areas  which  have 
usually  been  regarded  as  representing  the  original  myo- 
blasts, but  are  really  devoid  of  significance  as  cells,  the 
tissue  remaining,  strictly  speaking,  a  syncytium. 

The   Histogenesis   of   Striated   Muscle   Tissue.— The 
histogenesis  of  striated  or  voluntary  muscle  tissue  resembles 


«i^'^* 


Fig.    114. — Cross-section   of  a   Muscle  from   the  Thigh   of  a   Pig 

Embryo  75  mm.  Long. 

A,   Central   nucleus;   B,  new   peripheral   nucleus. —  {Macallum.) 

very  closely  that  which  has  just  been  described  for  the 
heart  muscle.  There  is  a  similar  formation  of  a  syncytium 
by  the  fusion  of  the  cells  of  the  myotomes,  an  appearance 
of  granules  which  unite  to  form  fibrils,  an  increase  of  the 
fibrils  by  longitudinal  division  and  a  primary  arrangement 
of  the  fibrils  around  the  periphery  of  areas  of  sarcoplasm 
(Fig.  114),  each  of  which  represents  a  muscle  fibre.  In 
addition  there  is  an  active  proliferation  of  the  nuclei  of  the 
original   myoblasts,   the   new   nuclei   arranging  themselves 


2o6       DEVELOPMENT  OF  SKELETAL  MUSCLES. 

more  or  less  regularly  in  rows  and  later  migrating  from 
their  original  central  position  to  the  periphery  of  the  fibers, 
and,  in  the  limb  muscles,  the  development  is  further  compli- 
cated by  a  process  of  degeneration  which  affects  groups  of 
muscle  fibers,  so  that  bundles  of  normal  fibers  are  separated 
by  strands  of  degenerated  tissue  in  which  the  fibrils  have 
disappeared,  the  nuclei  have  become  pale  and  the  sarcoplasm 
vacuolated  and  homogeneous.  Later  the  degenerated  tissue 
seems  to  disappear  entirely  and  mesenchymatous  connective 
tissue  grows  in  between  the  persisting-  fibers,  grouping  them 
into  bundles  and  the  bundles  into  the  individual  muscles. 

So  long  as  the  formation  of  new  fibrils  continues,  the 
increase  in  the  thickness  of  a  muscle  is  probably  due  to 
a  certain  extent  to  an  increase  in  the  actual  number  of 
fibers,  which  results  from  the  division  of  those  already  exist- 
ing. Subsequently,  however,  this  mode  of  g^rowth  ceases, 
the  further  increase  of  the  muscle  depending  upon  an  in- 
crease in  size  of  its  constituent  elements  (Macallum). 

The  Development  of  the  Skeletal  Muscles. — It  has 
alread}^  been  pointed  out  that  all  the  skeletal  muscles  of  the 
body,  with  the  exception  of  those  connected  with  the  bran- 
chial arches,  are  derived  from  the  myotomes  of  the  meso- 
dermic  somites,  even  the  limb  muscles  possibly  having  such 
an  origin,  although  the  cells  of  the  tissue  from  which  the 
muscles  of  the  limb  buds  form  lack  an  epithelial  arrange- 
ment and  are  indistinguishable  from  the  somatic  mesen- 
chyme which  forms  the  axial  cores  of  the  limbs. 

The  various  fibers  of  each  myotome  are  at  first  loosely 
arranged,  but  later  they  become  more  compact  and  are 
arranged  parallel  with  one  another,  their  long  axes  being 
directed  antero-posteriorly.  This  stage  is  also  transitory, 
however,  the  fibers  of  each  myotome  undergoing  various 
modifications  to  produce  the  conditions  existing  in  the  adult, 
in  which  the  original  segmental  arranoement  of  the  fibers 


DEVELOPMENT    OF    SKELETAL    MUSCLES.  20/ 

can  be  perceived  in  comparatively  few  muscles.  The  exact 
nature  of  these  modifications  is  almost  unknown  from  direct 
observation,  but  since  the  relation  between  a  nerve  and  the 
myotome  belonging  to  the  same  segment  is  established  at 
a  very  early  period  of  development  and  persists  throughout 
life,  no  matter  what  changes  of  fusion,  splitting,  or  migra- 
tion the  myotome  may  undergo,  it  is  possible  to  trace  out 
more  or  less  completely  the  history  of  the  various  myotomes 
by  determining  their  segmental  innervation.  It  is  known, 
for  example,  that  the  latissimus  dorsi  arises  from  the  seventh 
and  eighth*  cervical  myotomes,  but  later  undergoes  a  migra- 
tion, becoming  attached  to  the  lower  thoracic  and  lumbar 
vertebrae  and  to  the  crest  of  the  ilium,  far  away  from  its 
place  of  origin  (Mall),  and  yet  it  retains  its  nerve-supply 
from  the  seventh  and  eighth  cervical  nerves  with  which  it 
was  originally  associated,  its  nerve-supply  consec[uently  in- 
dicating the  extent  of  its  migration. 

By  following  the  indications  thus  afforded,  it  may  be  seen 
that  the  changes  which  occur  in  the  myotomes  may  be  re- 
ferred to  one  or  more  of  the  following  processes : 

1.  A  longitudinal  splitting  into  two  or  more  portions,  a 
process  well  illustrated  by  the  trapezius  and  sternomastoid, 
which  have  differentiated  by  the  longitudinal  splitting  of  a 
single  sheet  and  contain  therefore  portions  of  the  same  myo- 
tomes. The  sterno-hyoid  and  omohyoid  have  also  differ- 
entiated by  the  same  process,  and,  indeed,  it  is  of  frequent 
occurrence. 

2.  A  tangential  splitting  into  two  or  more  layers.  Ex- 
amples of  this  are  also  abundant  and  are  afforded  by  the 
muscles  of  the  fourth,  fifth,  and  sixth  layers  of  the  back, 
as  recognized  in  English  text-books  of  anatomy,  by  the  two 

*  This  enumeration  is  based  on  convenience  in  associating  the  myo- 
tomes with  the  nerves  which  supply  them.  The  myotomes  mentioned 
are  those  which  correspond  to  the  sixth  and  seventh  cervical  vertebrae. 


208       DEVELOPMENT  OF  SKELETAL  MUSCLES. 

oblique  and  the  transverse  layers  of  the  abdominal  walls,  and 
by  the  intercostal  mnscles  and  the  transversus  of  the  thorax. 

3.  A  fusion  of  portions  of  successive  myotomes  to  form 
a  single  muscle,  again  a  process  of  frecjuent  occurrence,  and 
well  illustrated  by  the  rectus  abdominis  (which  is  formed  by 
the  fusion  of  the  ventral  portions  of  the  last  six  or  seven 
thoracic  myotomes)  or  by  the  superficial  portions  of  the 
erector  spinse. 

4.  A  migration  of  parts  of  one  or  more  myotomes  over 
others.  An  example  of  this  process  is  to  be  found  in  the 
latissimus  dorsi,  whose  history  has  already  been  referred  to, 
and  it  is  also  beautifully  shown  by  the  serratus  anterior  and 
the  trapezius,  both  of  which  have  extended  far  beyond  the 
limits  of  the  segments  from  which  they  are  derived. 

5.  A  degeneration  of  portions  or  the  whole  of  a  myotome. 
This  process  has  played  a  very  considerable  part  in  the  evo- 
lution of  the  muscular  system  in  the  vertebrates.  When  a 
muscle  normally  degenerates,  it  becomes  converted  into  con- 
nective tissue,  and  many  of  the  strong  aponeurotic  sheets 
which  occur  in  the  body  owe  their  origin  to  this  process. 
Thus,  for  example,  the  aponeurosis  connecting  the  occipital 
and  frontal  portions  of  the  occipito-frontalis  is  due  to  this 
process  and  is  muscular  in  such  forms  as  the  lower  monkeys, 
and  a  good  example  is  also  to  be  found  in  the  aponeurosis 
which  occupies  the  interval  between  the  superior  and  inferior 
serrati  postici,  these  two  muscles  being  continuous  in  lower 
forms.  The  strong  lumbar  aponeurosis  and  the  aponeu- 
roses of  the  oblique  and  transverse  muscles  of  the  abdomen 
are  also  good  examples. 

Indeed,  in  comparing  one  of  the  mammals  with  a  mem- 
ber of  one  of  the  lower  classes  of  vertebrates,  the  greater 
amount  of  connective  tissue  compared  with  the  amount  of 
muscular  tissue  in  the  former  is  very  striking;,  the  inference 
being  that  these  connective-tissue  structures   (fascise,  apo- 


DEVELOPMENT    OF    SKELETAL    MUSCLES.  20g 

neuroses,  ligaments)  represent  portions  of  the  muscular 
tissue  of  the  lower  form  (Barclelehen).  Many  of  the  acces- 
sory ligaments  occurring  in  connection  with  diarthrodial 
joints  apparently  owe  their  origin  to  a  degeneration  of 
muscle  tissue,  the  fibular  lateral  ligament  of  the  knee-joint, 
for  instance,  being  probably  a  degenerated  portion  of  the 
peroneus  longus,  while  the  sacro-tuberous  ligament  ap- 
pears to  stand  in  a  similar  relation  to  the  long  head  of  the 
biceps  femoris  (Sutton). 

6.  Finally,  there  may  be  associated  with  any  of  the  first 
four  processes  a  change  in  the  direction  of  the  muscle-fibers. 
The  original  antero-posterior  direction  of  the  fibers  is  re- 
tained in  comparatively  few  of  the  adult  muscles  and  excel- 
lent examples  of  the  process  here  referred  to  are  to  be  found 
in  the  intercostal  muscles  and  the  muscles  of  the  abdominal 
walls.  In  the  musculature  associated  with  the  branchial 
arches  the  alteration  in  the  direction  of  the  fibers  occurs 
even  in  the  fishes,  in  which  the  original  direction  of  the 
muscle-fibers  is  very  perfectly  retained  in  other  myotomes, 
the  branchial  muscles,  however,  being  arranged  parallel  with 
the  branchial  cartilages  or  even  passing  dorso-ventrally  be- 
tween the  upper  and  lower  portions  of  an  arch,  and  so  form- 
ing what  may  be  regarded  as  a  constrictor  of  the  arch. 
This  alteration  of  direction  dates  back  so  far  that  the  con- 
strictor arrangement  may  well  be  taken  as  the  primary  con- 
dition in  studying  the  changes  which  the  branchial  muscu- 
lature has  undergone  in  the  mammalia. 

It  would  occupy  too  much  space  in  a  work  of  this  kind 
to  consider  in  detail  the  history  of  each  one  of  the  skeletal 
muscles  of  the  human  body,  but  a  statement  of  the  general 
plan  of  their  development  will  not  be  out  of  place.  For 
convenience  the  entire  systeili  may  be  divided  into  three 
portions — the  cranial,  trunk  and  limb  musculature;  and  of 
these,  the  trunk  musculature  may  first  be  considered. 
19 


2IO  THE    TRUNK    MUSCULATURE. 

The  Trunk  Musculature. — It  has  ah-eady  been  seen  (p. 
1 08)  that  the  myotomes  at  first  occupy  a  dorsal  position, 
becoming-  prolonged  ventrally  as  development  proceeds  so 
as  to  overlap  the  somatic  mesoderm,  until  those  of  opposite 
sides  come  into  contact  in  the  mid-ventral  line.  Before  this 
is  accomplished,  however,  a  longitudinal  splitting  of  each 
myotome  occurs,  whereby  there  is  separated  off  a  dorsal 
portion  which  gives  rise  to  a  segment  of  the  dorsal  muscu- 
lature of  the  trunk  and  is  supplied  by  the  ramus  dorsalis  of 
its  corresponding-  spinal  nerve.  In  the  lower  vertebrates 
this  separation  of  each  of  the  trunk  myotomes  into  a  dorsal 
and  ventral  portion  is  much  more  distinct  in  the  adult  than 
it  is  in  man,  the  two  portions  being  separated  by  a  horizontal 
plate  of  connective  tissue  extending  the  entire  length  of  the 
trunk  and  being  attached  by  its  inner  edge  to  the  transverse 
processes  of  the  vertebrse,  while  peripherally  it  becomes  con- 
tinuous with  the  connective  tissue  of  the  dermis  along  a 
line  known  as  the  lateral  line.  In  man  the  dorsal  portion 
is  also  much  smaller  in  proportion  to  the  ventral  portion 
than  in  the  lower  vertebrates.  From  this  dorsal  portion  of 
the  myotomes  are  derived  the  muscles  belonging  to  the  three 
deepest  layers  of  the  dorsal  musculature,  the  more  super- 
ficial layers  being  composed  of  muscles  belonging  to  the 
limb  system.  Further  longitudinal  and  tangential  divisions 
and  a  fusion  of  successive  myotomes  bring  about  the  condi- 
tions which  obtain  in  the  adult  dorsal  musculature. 

While  the  myotomes  are  still  some  distance  from  the  mid- 
ventral  line  another  longitudinal  division  affects  their  ventral 
edges  (Fig.  115),  portions  being  thus  separated  which  later 
fuse  more  or  less  perfectly  to  form  longitudinal  bands  of 
muscle,  those  of  opposite  sides  being  brought  into  apposition 
along  the  mid-ventral  line  by  the  continued  growth  ventrally 
of  the  myotomes.  In  this  way  are  formed  the  rectus  and 
pyramidalis  muscles  of  the  abdomen  and  the  depressors  of 


,  THE    TRUNK    MUSCULATURE.  211 

the  hyoid  bone,  the  genio-hyoid  and  genio-glossus*  in  the 
neck  region.  In  the  thoracic  region  this  rectus  set  of  mus- 
cles, as  it  may  be  termed,  is  not  represented  except  as  an 
anomaly,  its  absence  being  probably  correlated  with  the 
development  of  the  sternum  in  this  region. 

The  lateral  portions  of  the  myotomes  which   intervene 


Fig.  115. — Embryo  of  13  mm.  showing  the  Formation  of  the  Rectus 
Muscle.— (Ma//.) 

between  the  dorsal  and  rectus  muscles  divide  tangentially, 
producing  from  their  dorsal  portions  in  the  cervical  and 
lumbar  regions  muscles,  such  as  the  longus  capitis  and  colli 


*  This  muscle  is  supplied  by  the  hypoglossal  nerve,  but  for  the  present 
purpose  it  is  convenient  to  regard  this  as  a  spinal  nerve,  as  indeed  it 
primarily  is. 


212  THE    TRUNK    MUSCULATURE, 

and  the  psoas,  which  He  beneath  the  vertebral  column  and 
hence  have  been  termed  hyposkeletal  muscles  (Huxley). 
More  ventrally  three  sheets  of  muscles,  lying  one  above  the 
other,  are  formed,  the  fibers  of  each  sheet  being  arranged  in 
a  definite  direction  differing  from  that  found  in  the  other 
sheets.  In  the  abdomen  there  are  thus  formed  the  two 
oblicjue  and  the  transverse  muscles,  in  the  thorax  the  inter- 
costals  and  the  transversus  thoracis,  while  in  the  neck  these 
portions  of  some  of  the  myotomes  disappear,  those  of  the 
remainder  giving  rise  to  the  scaleni  muscles,  portions  of  the 
trapezius  and  sternomastoid  (Bolk),  and  possibly  the  hyo- 
glossus  and  styloglossus.  In  the  abdominal  region,  and  to 
a  considerable  extent  in  the  neck  also,  the  various  portions 
of  myotomes  fuse  together,  but  in  the  thorax  they  retain  in 
the  intercostals  their  original  distinctness,  being  separated 
by  the  ribs. 

The  table  on  page  213  will  show  the  relation  of  the 
various  trunk  muscles  to  the  portions  of  the  myotomes. 

The  intimate  association  between  the  pelvic  girdle  and 
the  axial  skeleton  brings  about  extensive  modifications  of 
the  posterior  trunk  myotomes.  So  far  as  their  dorsal  por- 
tions are  concerned  probably  all  these  myotomes  as  far 
back  as  the  fifth  sacral  are  represented  in  the  sacro-spinalis, 
but  the  ventral  portions  from  the  first  lumbar  myotome  on- 
wards are  greatly  modified.  The  last  myotome  taking  part 
in  the  formation  of  the  rectus  abdominis  is  the  twelfth 
thoracic  and  the  last  to  be  represented  in  the  lateral  muscu- 
lature of  the  abdomen  is  the  first  lumbar,  the  ventral  por- 
tions of  the  remaining  lumbar  and  of  the  first  and  second 
sacral  myotomes  either  having  disappeared  or  being  devoted 
to  the  formation  of  the  musculature  of  the  lower  limb. 

The  ventral  portions  of  the  third  and  fourth  sacral  myo- 
tomes are  represented,  however,  by  the  levator  ani  and  coccy- 
geus,  and  are  the  last  myotomes  which  persist  as  muscles 


THE    TRUNK    MUSCULATURE. 


213 


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214 


THE    TRUNK    MUSCULATURE. 


ill  the  human  body,  although  traces  of  still  more  posterior 
myotomes  are  to  be  found  in  muscles  such  as  the  curvator 
coccygis  sometimes  developed  in  connection  with  the  coccy- 
geal vertebrae. 

The  perineal  muscles  and  the  external  sphincter  ani  are 
also  developments  of  the  third  and  fourth  (and  second) 
sacral  myotomes.  At  a  time  when  the  cloaca  (see  p.  297) 
is  still  present,  a  sheet  of  muscles  lying  close  beneath  the 
integument  forms  a  sphincter  around  its  opening  (Fig.  116). 


Fig.  116.— Perineal  Region  of  Embryos  of  (A)  Two  Months  and 
(B)  Four  to  Five  Months,  showing  the  Development  of  the 
Perineal  Muscles. 

dc,  Nervus  dorsalis  clitoridis  ;  p,  pudendal  nerve ;  sa,  sphincter  ani ;  sc, 
sphincter  cloacae;  sv,  sphincter  vaginje. —  (Popozvsky.) 


On  the  development  of  the  partition  which  divides  the 
cloaca  into  rectal  and  urinogenital  portions,  the  sphincter  is 
also  divided,  its  more  posterior  portion  persisting  as  the 
external  sphincter  ani,  while  the  anterior  part  gradually 
differentiates  into  the  various  perineal  muscles  (Popowsky). 
The  Cranial  Musculature. — As  was  pointed  out  in  an 
earlier  chapter,  the  existence  of  distinct  mesodermic  somites 
has  not  yet  been  completely  demonstrated  in  the  head  of  the 
human  embryo, but  in  lower  forms,  such  as  the  elasmobranch 
fishes,  they  are  clearly  distinguishable,  and  it  may  be  sup- 


THE    CRANIAL    MUSCULATURE.  215 

posed  that  their  indistinctness  in  man  is  a  secondary  con- 
dition. Exactly  how  many  of  these  somites  are  represented 
in  the  mammahan  head  it  is  impossible  to  say,  but  it  seems 
probable,  from  comparison  with  lower  forms,  that  there  is 
a  considerable  number.  The  majority  of  them,  however, 
early  underg-o  degeneration,  and  in  the  adult  condition  only 
three  are  recognizable,  two  of  which  are  praeoral  in  position 
and  one  postoral.  The  myotomes  of  the  anterior  praeoral 
segment  give  rise  to  the  muscles  of  the  eye  supplied  by  the 
third  cranial  nerve,  those  of  the  posterior  one  furnish  the 
superior  oblique  muscles  innervated  by  the  fourth  nerve, 
while  from  the  postoral  myotomes  the  lateral  recti,  supplied 
by  the  sixth  nerve,  are  developed.  The  muscles  supplied 
by  the  hypoglossal  nerve  are  also  derived  from  myotomes, 
but  they  have  already  been  considered  in  connection  wnth 
the  trunk  musculature. 

The  remaining  muscles  of  the  head  differ  from  all  other 
voluntary  muscles  of  the  body  in  the  fact  that  they  are 
derived  from  the  branchiomeres  formed  by  the  segmentation 
of  the  cephalic  ventral  mesoderm.  These  muscles,  there- 
fore, are  not  to  be  regarded  as  equivalent  to  the  myotomic 
muscles  if  their  embryological  origin  is  to  be  taken  as  a 
criterion  of  equivalency,  and  in  their  case  it  would  seem, 
from  the  fact  that  they  are  innervated  by  nerves  funda- 
mentally distinct  from  those  which  supply  the  myotomic 
mucles,  that  this  criterion  is  a  good  one.  They  must  be 
regarded,  therefore,  as  belonging  to  a  special  category,  and 
may  be  termed  hranchionicric  muscles  to  distinguish  them 
from  the  myotomic  set. 

If  their  embryological  origin  be  taken  as  a  basis  for  homology, 
it  is  clear  that  they  should  be  regarded  as  equivalent  to  the 
muscles  derived  from  the  ventral  mesoderm  of  the  trunk,  and 
these,  as  has  been  seen,  are  the  non-striated  muscles  associated 
with  the  viscera,  among  which  may  be  included  the  striated 
heart  muscle.     At  first  sight  this  homology  seems  decidedly 


2l6  THE    CRANIAL    MUSCULATURE. 

strained,  chiefly  because  long-continued  custom  has  regarded 
the  histological  and  physiological  peculiarities  of  striated  and 
non-striated  muscle  tissue  as  fundamental.  It  may  be  pointed 
out,  however,  that  the  branchiomeric  muscles  are,  strictly  speak- 
ing, visceral  muscles,  and  indeed  give  rise  to  muscle  sheets 
(the  constrictors  of  the  pharynx)  which  surround  the  upper 
or  pharyngeal  portion  of  the  digestive  tract.  It  is  possible, 
then,  that  the  homology  is  not  so  strained  as  might  appear, 
but  further  discussion  of  it  may  profitably  be  deferred  until 
the  cranial  nerves  are  under  consideration. 

The  skeleton  of  the  first  l:)ranchial  arch  becomes  converted 
partly  into  the  jaw  apparatus  and  partly  into  auditory 
ossicles,  and  the  muscles  derived  from  the  corresponding 
branchiomere  become  the  muscles  of  mastication  (the  tem- 
poral, masseter,  and  pterygoids),  the  mylohyoid,  anterior 
belly  of  the  dig"astric,  the  tensor  veli  palatini  and  the  tensor 
tympani.  The  nerve  which  corresponds  to  the  first  bran- 
chial arch  is  the  trigeminus  or  fifth,  and  consequently  these 
various  muscles  are  supplied  by  it. 

The  second  arch  has  corresponding  to  it  the  seventh 
nerve,  and  its  musculature  is  partly  represented  by  the  stylo- 
hyoid and  posterior  belly  of  the  digastric  and  by  the  stape- 
dius muscle  of  the  middle  ear.  From  the  more  superficial 
portions  of  the  branchiomere,  however,  a  sheet  of  tissue 
arises  which  gradually  extends  tipw^ard  and  downward  to 
form  a  thin  covering-  for  the  entire  head  and  neck,  its  lower 
portion  giving-  rise  to  the  platysma  and  the  nuchal  fascia 
which  extends  backward  from  the  dorsal  border  of  this 
muscle,  while  its  upper  parts  become  the  occipito-frontalis 
and  the  superficial  muscles  of  the  face  (the  muscles  of  ex- 
pression), together  with  the  fascise  wdiich  unite  the  various 
muscles  of  this  group.  The  extension  of  the  platysma  sheet 
of  muscles  over  the  face  is  well  shown  b}^  the  development 
of  the  branches  of  the  facial  nerve  which  supply  it  (Fig.  117). 

The  degeneration  of  the  upper  part  of  the  third  arch 
produces  a  shifting  forward  of  one  of  the  muscles  derived 


-THE    CRANIAL    MUSCULATURE, 


217 


Fig.  117. — Head  of  Embryos  (A)  of  Two  Months  and  (B)  of  Three 
Months  showing  the  Extension  of  the  Seventh  Nerve  upon 
THE  Face. —  {Po[>ozvsky.) 


2l8  THE    CRANIAL    MUSCULATURE. 

from  its  branchiomere,  the  stylopharyngeus  arising  from 
the  base  of  the  styloid  process.  The  innervation  of  this 
muscle  by  the  ninth  nerve  indicates,  however,  its  true  sig- 
nificance, and  since  fibers  of  this  nerve  of  the  third  arch  also 
pass  to  the  constrictor  muscles  of  the  pharynx,  a  portion  of 
these  must  also  be  regarded  as  having  arisen  from  the  third 
branchiomere. 

The  cartilages  of  the  fourth  and  fifth  arches  enter  into 
the  formation  of  the  larynx  and  the  muscles  of  the  corre- 
sponding branchiomeres  constitute  the  muscles  of  the  larynx, 
together  with  the  remaining  portions  of  the  constrictors  of 
the  pharynx  and  the  muscles  of  the  soft  palate,  with  the 
exception  of  the  tensor.  Both  these  arches  have  branches 
of  the  tenth  nerve  associated  with  them  and  hence  this  nerve 
supplies  the  muscles  named.  In  addition,  two  of  the  ex- 
trinsic muscles  of  the  tongue,  the  glosso-palatinus  and  chon- 
droglossus,  belong  to  the  fourth  or  fifth  branchiomere, 
although  the  remaining  muscles  of  this  physiological  set  are 
myotomic  in  origin. 

Finally,  portions  of  two  other  muscles  should  probably 
be  included  in  the  list  of  branchiomeric  muscles,  these  mus- 
cles being  the  trapezius  and  sternomastoid.  It  has  already 
been  seen  that  they  are  partly  derived  from,  the  cervical 
myotomes,  but  they  also  appear  to  be  innervated  in  part  by 
the  spinal  accessory,  and  since  the  motor  fibers  of  this  nerve 
are  serially  homologous  with  those  of  the  vagus  it  would 
seem  that  the  muscles  which  they  supply  are  probably 
branchiomeric  in  origin.  Observations  on  the  development 
of  these  muscles,  determining  their  relations  to  the  branch- 
iomeres, are  necessary,  however,  before  their  morphological 
significance  can  be  regarded  as  definitely  settled. 

The  table  on  page  219  shows  the  relations  of  the  various 
cranial  muscles  to  the  myotomes  and  branchiomeres,  as 
well  as  to  the  motor  cranial  nerves. 


THE    CRANIAL    MUSCULATURE. 


2  19 


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220  THE    LIMB    MUSCLES. 

The  Limb  Muscles. — It  has  been  customary  to  regard 
the  hmb  muscles  as  derivatives  of  certain  of  the  myotomes, 
these  structures  in  their  growth  ventrally  in  the  trunk  wahs 
being  supposed  to  pass  out  upon  the  postaxial  surface  of 
the  Hml:)  buds  and  loop  back  again  to  the  trunk  along  the 
prpeaxial  surface,  each  myotome  thus  giving  rise  to  a  por- 
tion of  both  the  dorsal  and  the  ventral  musculature  of  the 
limb.  This  view  has  not,  however,  been  verified  by  direct 
observation  of  an  actual  looping  of  the  myotomes  over  the 
axis  of  the  limb  buds ;  indeed,  on  the  contrary,  the  limb 
muscles  have  been  found  to  develop  from  the  cores  of 
mesenchyme  which  form  the  axes  of  the  limb  buds  and  from 
which  the  limb  skeleton  is  also  developed.  This  may  be 
explained  by  supposing  that  the  limb  muscles  are  primarily 
derivatives  of  the  myotomes  and  that  an  extensive  concen- 
tration of  their  developmental  history  has  taken  place,  so 
that  the  axial  mesenchyme  actually  represents  myotomic 
material  even  though  no  direct  connection  between  it  and 
the  myotomes  can  be  discovered.  Condensations  of  the 
developmental  history  certainly  occur  and  the  fact  that  the 
muscles  of  the  human  limbs,  as  they  differentiate  from  the 
axial  cores,  present  essentially  the  same  arrangement  as  in 
the  adult  seems  to  indicate  that  there  is  actually  an  extensive 
condensation  of  the  phylogenetic  history  of  the  individual 
muscles,  since  comparative  anatomy  shows  the  arrangement 
of  the  muscles  of  the  higher  mammalian  limbs  to  be  the 
result  of  a  long  series  of  progressive  modifications  from  a 
primitive  condition.  However,  even  though  this  be  the 
case,  there  is  yet  the  possibility  that  the  limb  musculature, 
like  the  limb  skeleton,  may  take  its  origin  from  the  ventral 
mesoderm  and  consequently  belong  to  a  different  embry- 
onic category  from  the  axial  myotomic  muscles. 

The  strongest  evidence  in  favor  of  the  myotomic  origin 
of  tlic  liml)  muscles  is  that  furnished  by  their  nerve  supply, 


THE    LIMB    MUSCLES.  221 

this  presenting  a  distinctl}^  segmental  arrangement.  This 
does  not  necessarily  imply,  however,  a  corresponding  pri- 
marily metameric  arrangement  of  the  muscles,  any  more 
than  the  pronouncedly  segmental  arrangement  of  the  cuta- 
neous nerves  implies  a  primary  metamerism  of  the  dermis 
(see  p.  148).  It  may  mean  only  that  the  nerves,  being  seg- 
mental, retain  their  segmental  relations  to  one  another  even 
in  their  distribution  to  non-metameric  structures,  and  that, 
consecjuently,  the  limb  musculature  is  supplied  in  succes- 
sion from  one  border  of  the  limb  bud  to  the  other  from 
succeeding  nerve  roots. 

But  whether  further  observation  may  prove  or  disprove 
the  myotomic  origin  of  the  limb  musculature,  the  fact  re- 
mains that  it  possesses  a  segmentally  arranged  innervation, 
and  it  is  possible,  therefore,  to  recognize  in  the  limb  buds 
a  series  of  parallel  bands  of  muscle  tissue,  extending  longi- 
tudinally along  the  bud  and  each  supplied  by  a  definite  seg- 
mental nerve.  And  furthermore,  corresponding  to  each 
band  upon  the  ventral  (prseaxial)  surface  of  the  limb  bud, 
there  is  a  band  similarly  innervated  upon  the  dorsal  (post- 
axial)  surface,  since  the  fibers  which  pass  to  the  limb  from 
each  nerve  root  sooner  or  later  arrange  themselves  in  pr^e- 
axial  and  postaxial  groups  as  is  shown  in  the  diag'ram  Fig. 
118.  The  first  nerve  which  enters  the  limb  bud  lies  along 
its  anterior  border,  and  consequently  the  muscle  bands 
which  are  supplied  by  it  will,  in  the  adult,  lie  along  the 
outer  side  of  the  arm  and  along  the  inner  side  of  the  leg, 
in  consecjuence  of  the  rotation  in  opposite  directions  which 
the  limbs  undergo  during  development  (see  p.  91). 

The  first  nerve  which  supplies  the  muscles  attached  to 
the  dorsum  of  the  ilium  is  the  second  lumbar,  and  follow- 
ing that  there  come  successively  from  before  backward  the 
remaining  lumbar  and  the  first  and  second  sacral  nerves. 
The  arrangement  of  the  muscle  bands  supplied  by  these 


222 


THE    LIMB    MUSCLES. 


nerves  and  the  muscles  of  the  adult  to  which  they  contribute 
may  be  seen  from  Fig.  119.  What  is  shown  there  is  only 
the  upper  portions  of  the  post-axial  bands,  their  lower  por- 
tions extending  downward  on  the  anterior  surface  of  the 
leg.  Only  the  sacral  bands,  however,  extend  throughout 
the  entire  length  of  the  limb  into  the  foot,  the  second  lum- 


dm^ 


trd 


trv  ■ 


Fig.  118. — Diagram  of  a  Segment  of  the  Body  and  Limb. 
bl.  Axial  blastema ;  dm,  dorsal  musculature  of  trunk ;  rl,  nerve  to  limb ; 
s,  septum  between  dorsal  and  ventral  trunk  musculature;  str.d, 
dorsal  laj'er  of  limb  musculature;  tr.d  and  tr.v,  dorsal  and  ventral 
divisions  of  a  spinal  nerve ;  vni,  ventral  musculature  of  the  trunk. — 
(KoUmann.) 

bar  band  passing  down  only  to  about  the  middle  of  the 
thigh,  the  third  to  about  the  knee,  the  fourth  to  about  the 
middle  of  the  crus  and  the  fifth  as  far  as  the  base  of  the 
fifth  metatarsal  bone,  and  the  same  is  true  of  the  corre- 
sponding praeaxial  bands,  which  descend  from  the  ventral 
surface  of  the  os  coxae  (innominatum)  along  the  inner  and 
posterior  surfaces  of  the  leg  to  the  same  points.     The  first 


THE    LIMB    MUSCLES. 


223 


and  second  sacral  bands  can  be  traced  into  tbe  foot,  the  first 
giving  rise  to  the  musculature  of  its  inner  side  and  the  sec- 


FiG.  119. — External  Surface  of  the  Os  Innominatum  showing  the 
Attachment  of  Muscles  and  the  Zones  Supplied  by  the  Various 
Nerves. 

12,  Twelfth  thoracic  nerve ;  I  to  V,  lumbar  nerves ;  i  and  2,  sacral 
nerves. —  (Bolk.) 

ond  to  that  of  its  outer  side,  the  pr^eaxial  bands  forming 
the  plantar  musculature,  while  the  postaxial  ones  are  upon 


2  24 


THE    LIMB    MUSCLES. 


the  dorsum  of  the  foot  as  a  result  of  the  rotation  which  the 
limb  has  underg-one. 

In  a  transverse  section  through  a  limb  at  any  level  all 


Fig.    I20. — Sections   through    (A)    the  Thigh   and    (B)    the   Calf 

SHOWING    THE    ZONES    SUPPLIED    BY    THE    NeRVES.       ThE    NeRVES    ARE 

Numbered   in    Continuation    with    the   Thoracic    Series. —  (A, 
after  Bolk.) 

the  muscle  bands,  both  pr?eaxial  and  postaxial,  which  de- 
scend to  that  level  will  be  cut  and  will  lie  in  a  definite  suc- 
cession from  one  border  of  the  limb  to  the  other,  as  is  seen 


THE    LIMB    MUSCLES. 


225 


in  Fig-.  120.  In  the  dififerentiation  of  the  individual  mus- 
cles which  proceeds  as  the  nerves  extend  from  the  trunk 
into  the  axial  mesenchyme  of  the  limb,  the  muscle  bands 
underg-o  modifications  similar  to  those  already  described 
as  occurring  in  the  trunk  myotomes.  Thus,  each  of  the 
muscles  represented  in  Fig.  120,  B,  is  formed  by  the  fusion 
of  elements  derived  from  two  or  more  bands ;  the  soleus 
and    gastrocnemius    represent    deep    and    superficial    layers 


Fig.   121. — Section  through  the  Upper  Part  of  the  Arm    showing 

THE  Zones  Supplied  by  the  Nerves. 

Sv  to  yv,  Ventral  branches ;  sd  to  M,  dorsal  branches  of  the  cervical 

nerves. —  (Bolk.) 

formed  from  the  same  bands  by  a  horizontal  (tangential) 
splitting,  these  same  muscles  contain  a  portion  of  the  second 
sacral  band  which  overlaps  muscles  composed  only  of  higher 
myotomes,  and  the  intermuscular  septum  between  the  pero- 
neus  brevis  and  the  flexor  hallucis  longus  represents  a  por- 
tion of  the  third  sacral  band  which  has  degenerated  into 
connective  tissue. 

A  similar  arrangement  occurs  in  the  bands  which  are 
to  be  recognized  in  the  musculature  of  the  upper  limb. 
These  are  supplied  by  the  fourth,  fifth,  sixth,  seventh  and 


226  THE    LIMB    MUSCLES. 

eighth  cervical  and  the  first  thoracic  nerves,  and  only  those 
supplied  by  the  eighth  cervical  and  the  first  thoracic  nerves 
extend  as  far  as  the  tips  of  the  fingers.  The  arrangement 
of  the  bands  in  the  upper  part  of  the  brachium  may  be 
seen  from  Fig.  121,  in  connection  with  which  it  must  be 
noted  that  the  fourth  cervical  band  does  not  extend  down 
to  the  level  at  which  the  section  is  taken  and  that  the  prse- 
axial  band  of  the  eighth  cervical  nerve  and  both  the  prce- 
axial  and  postaxial  bands  of  the  first  thoracic  are  repre- 
sented only  by  connective  tissue  in  this  region. 

In  another  sense  than  the  longitudinal  one  there  is  a 
division  of  the  limb  musculature  into  more  or  less  definite 
areas,  namely,  in  a  transverse  direction  in  accordance 
with  the  jointing  of  the  skeleton.  Thus,  there  may  be 
recognized  a  group  of  muscles  which  pass  from  the  axial 
skeleton  to  the  pectoral  girdle,  another  from  the  limb 
girdle  to  the  brachium  or  thigh,  another  from  the  brachium 
or  thigh  to  the  antibrachium  or  crus,  another  from  the 
antibrachium  or  crus  to  the  carpus  or  tarsus,  and  another 
from  the  carpus  or  tarsus  to  the  digits.  This  transverse 
segmentation,  if  it  may  be  so  termed,  is  not,  however, 
perfectly  definite,  many  muscles,  even  in  the  lower  verte- 
brates, passing  over  more  than  one  joint,  and  in  the  mam- 
malia, especially,  it  is  further  obscured  by  secondary  migra- 
tions, by  the  partial  degeneration  of  muscles  and  by  an  end 
to  end  union  of  primarily  distinct  muscles. 

The  latissimus  dorsi,  serratus  anterior  and  pectoral 
muscles  are  all  examples  of  a  process  of  migration  as  is 
shown  by  their  innervation  from  cervical  nerves,  as  well 
as  by  the  actual  migration  which  has  been  traced  in  the 
developing  embryo  (Mall,  Lewis).  In  the  lower  limb 
evidences  of  migration  may  be  seen  in  the  femoral  head  of 
the  biceps,  comparative  anatomy  showing  this  to  be  a  de- 
rivative of  the  gluteal  set  of  muscles  which  has  secondarily 


THE    LIMB    MUSCLES.  22/ 

become  attached  to  the  femur  and  has  associated  itself 
with  a  prseaxial  muscle  to  form  a  compound  structure. 
An  appearance  of  migration  may  also  be  produced  by  a 
muscle  making  a  secondary  attachment  below  its  original 
origin  or  above  the  insertion  and  the  upper  or  lower  part,  as 
the  case  may  be,  then  degenerating  into  connective  tissue. 
This  has  been  the  case  with  the  peroneus  longus,  which,  in 
the  lower  mammals,  has  a  femoral  origin,  but  has  in  man  a 
new  origin  from  the  fibula,  its  upper  portion  being  repre- 
sented by  the  fibular  lateral  ligament  of  the  knee-joint. 
So  too  the  pectoralis  minor  primarily  inserted  into  the 
humerus,  but -it  has  made  a  secondary  attachment  to  the 
coracoid  process,  its  distal  portion  forming  a  coraco-humeral 
ligament. 

The  comparative  study  of  the  flexor  muscles  of  the  anti- 
brachial  and  crural  regions  has  yielded  abundant  evidence 
of  extensive  modifications  in  the  differentiation  of  the 
limb  muscles.  In  the  tailed  amphibia  these  muscles  are 
represented  by  a  series  of  superposed  layers,  the  most 
superficial  of  which  arises  from  the  humerus  or  femur, 
while  the  remaining  ones  take  their  origin  from  the  ulna 
or  fibula  and  are  directed  distally  and  laterally  to  be  in- 
serted either  into  the  palmar  or  plantar  aponeurosis,  or, 
in  the  case  of  the  deeper  layers,  into  the  radius  (tibia)  or 
carpus  (tarsus).  In  the  arm  of  the  lower  mammalia  the 
deepest  layer  becomes  the  pronator  cjuadratus,  the  lateral 
portions  of  the  superficial  layer  are  the  flexor  carpi  ulnaris 
and  the  flexor  carpi  radialis,  while  the  intervening  layers, 
together  with  the  median  portion  of  the  superficial  one, 
assuming  a  more  directly  longitudinal  direction,  fuse  to 
form  a  common  flexor  mass  which  acts  on  the  digits  through 
the  palmar  aponeurosis.  From  this  latter  structure  and 
from  the  carpal  and  metacarpal  bones  five  layers  of  palmar 
muscles  take  origin.     The  radial  and  ulnar  portions  of  the 


228 


THE    LIMB    MUSCLES. 


most  superficial  of  these  become  the  flexor  polHcis  brevis 
and  abductor  poflicis  and  the  abductor  quinti  digiti,  while 
the  rest  of  the  layer  degenerates  into  connective  tissue, 
forming    tendons    which   pass     to   the    four   ulnar    digits. 


Fig.  122. — Transverse  sections  through  (A)  the  forearm  and  (B)  the 
hand  showing  the  arrangement  of  the  layers  of  the  flexor  muscles. 
The  superficial  layer  is  shaded  horizontally,  the  second  layer  verti- 
cally, the  third  obliquely  to  the  left,  the  fourth  vertically,  and  the 
fifth  obliquely  to  the  right.  AbM,  abductor  digiti  quinti;  AdP, 
adductor  pollicis ;  BR,  brachio-radialis ;  ECD,  extensor  digitorum 
communis;  ECU,  e.xtensor  carpi  ulnaris ;  EI,  extensor  indicis; 
EMD,  e.xtensor  digiti  quinti ;  EMP,  abductor  pollicis  longus ;  ERB 
extensor  carpi  radialis  brevis;  FCR.  flexor  carpi  radialis;  ECU, 
flexor  carpi  ulnaris ;  FLP,  flexor  pollicis  longus ;  EM,  flexor  digiti 
quinti  brevis;  PP.  flexor  digitorum  profundus;  ES,  flexor  digi- 
torum sublimis;  ID,  interossei  dorsales;  /[',  interossei  volares ; 
L,  lumbricales;  OM,  opponens  digiti  quinti;  PE,  palmaris  longus; 
PT,  pronator  teres;  R,  radius;  U,  ulna;  II-V,  second  to  fifth  meta- 
carpal. 


THE    LIMB    MUSCLES.  229 

Gradually  superficial  portions  of  the  antibrachial  flexor 
mass  separate  ofif,  carrying-  with  them  the  layers  of  the 
palmar  aponeurosis  from  which  the  tendons  representing 
the  superficial  layer  of  the  palmar  muscles  arise,  and  they 
form  with  these  the  flexor  digitorum  sublimis.  The 
deeper  layers  of  the  antibrachial  flexor  mass  become  the 
flexor  digitorum  profundus  and  the  flexor  pollicis  longus 
(Fig.  122,  A),  and  retain  their  conection  with  the  deeper 
layers  of  the  palmar  aponeurosis  which  form  their  tendons; 
and  since  the  second  layer  of  the  palmar  muscles  takes 
origin  from  this  portion  of  the  aponeurosis  it  becomes  the 
lumbrical  muscles,  arising  from  the  profundus  tendons 
(Fig.  122,  B).  The  third  layer  of  palmar  muscles  be- 
comes the  adductors  of  the  digits,  reduced  in  man  to  the 
adductor  pollicis,  while  from  the  two  deepest  layers  the 
interossei  are  developed.  Of  these  the  fourth  layer  con- 
sists primarily  of  a  pair  of  slips  corresponding  to  each 
digit,  while  the  fifth  is  represented  by  a  series  of  muscles 
which  extend  obliquely  across  between  adjacent  metacarpals. 
With  these  last  muscles  certain  of  the  fourth  layer  slips 
unite  to  form  the  dorsal  interossei,  while  the  rest  become 
the  volar  interossei. 

The  modifications  of  the  almost  identical  primary  ar- 
rangement in  the  crus  and  foot  are  somewhat  different. 
The  superficial  layer  of  the  crural  flexors  becomes  the  gas- 
trocnemius and  plantaris  (Fig.  123,  A)  and  the  deepest 
layer  becomes  the  popliteus  and  the  interosseous  membrane. 
The  second  and  third  layers  unite  to  form  a  common  mass 
which  is  inserted  into  the  deeper  layers  of  the  plantar  apo- 
neurosis and  later  differentiates  into  the  soleus  and  the  long 
digital  flexor,  the  former  shifting  its  insertion  from  the 
plantar  aponeurosis  to  the  os  calcis,  while  the  flexor  retains 
its  connection  with  the  deeper  layers  of  the  aponeurosis, 
these  separating  from  the  superficial  layer  to  form  the  long 


230 


THE    LIMB    MUSCLES. 


flexor  tendons.  The  fourth  layer  partly  assumes  a  longitu- 
dinal direction  and  becomes  the  tibialis  posterior  and  the 
flexor  hallucis  longus  and  partly  retains  its  original  oblique 
direction  and  its  connection  with  the  deep  layers  of  the 
plantar  aponeurosis,  becoming  the  quadratus  plantse.  In 
the  foot  (Fig.  123,  B)  the  superficial  layer  persists  as  mus- 
cular tissue,   forming  the  abductors,  the  flexor  digitorum 


Fig.  123. — Transverse  sections  through  (A)  the  crus  and  (B)  the  foot, 
showing  the  arrangement  of  the  layers  of  the  flexor  muscles.  The 
shading  has  the  same  signihcance  as  in  the  preceding  figure.  AbH , 
abductor  hallucis;  AbM,  abductor  minimi  digiti ;  AdH,  adductor 
hallucis ;  ELD,  extensor  longus  digitorum ;  F,  fibula ;  FBD,  flexor 
brevis  digitorum ;  FBH ,  flexor  brevis  hallucis ;  FBM,  flexor  brevis 
minimi  digiti;  FLD,  flexor  longus  digitorum;  G,  gastrocnemius; 
IDj  interossei  dorsalis;  IV,  interossei  ventrales ;  L,  lumbricales ; 
P,  plantaris ;  Pc,  peroneus  longus;  Po,  popliteus;  S,  soleus;  T, 
tibia;  TA,  tibialis  anticus;  TP,  tibialis  posticus;  I-V,  first  to  fifth 
metatarsal. 


brevis  and  the  medial  head  of  the  flexor  hallucis  brevis,  the 
second  and  third  layers  become  respectively  the  lumbricales, 
the  lateral  head  of  the  flexor  hallucis  brevis  and  the  adduc- 
tor hallucis,  while  the  fourth  and  fifth  layers  together  form 
the  interossei,  as  in  the  hand,  the  flexor  quinti  digiti  brevis 
really  belonging  to  that  group  of  muscles. 


LITERATURE.  23 1 


LITERATURE. 


/  C.  R.  Bardeen  and  W.  H.  Lewis  :  "  Development  of  the  Limbs,  Body- 
wall,  and  Back    in  Man,"  The  American  Journal  of  Anat.,  i,  1901. 
K.  Bardeleben  :  "  Muskel  und  Fascia,"  Jenaische  Zcitschr.  fiir  Natiir- 

wissensch.,  xv,  1882. 
L.  Bolk  :  "  Beziehungen  zwischen  Skelett,  Muskulatur  und  Nerven  der 
Extremitaten,    dargelegt   am   Beckengiirtel,   an   dessen    Muskulatur 
sowie  am  Plexus  lumbosacralis,"  Morphol.  Jahrbuch,  xxi,  1894. 
L.   Bolk  :   "  Rekonstruktion  der  Segmentirung  der  Gliedmassenmusku- 
latur    dargelegt    an    den    Muskeln    des     Oberschenkels     und    des 
Schultergiirtels,"  Morphol.   Jahrbuch,   xxii,   1895. 
L.  Bolk  :  "  Die  Sklerozonie  des  Humerus,"  Morphol.  Jahrbuch,  xxiii, 
1896. 
\/    L.    Bolk  :    "  Die   Segmentaldifferenzierung   des   menschlichen   Rumpfes 

und  seiner  Extremitaten,"  i,  Morphol.  Jahrbuch,  xxv,  1898. 
K    R.   FuTAMURA :   "  Ueber  die  Entwickelung  der  Facialismuskulatur  des 

Menschen,"  Anat.  Hefte,  xxx,  1906. 
^    E.  GoDLEWSKi":  "Die  Entwicklung  des  Skelet-  und  Herzmuskelgewebes 
der  Saugethiere,"  Archiv  filr  mikr.  Anat.,  lx,   1902. 
W.  P.  Herringham  :  "  The  Minute  Anatomy  of  the  Brachial  Plexus," 
Proceedings  of  the  Royal  Soc.  London,  xli,  1886. 
^'    W.  H.  Lewis  :  "  The  Development  of  the  Arm  in  Man,"  Amer.  Jour,  of 

Anat.,  I,  1902. 
^     J.  B.  MacCallum  :  "  On  the  Histology  and  Histogenesis  of  the  Heart 

Muscle-cell,"  Anat.  Anseigcr,  xiii,  1897. 
V     J.  B.  MacCallum  :  "  On  the  Histogenesis  of  the  Striated  Muscle-fiber 
and  the  Growth  of  the  Human  Sartorius  Muscle,"  Johns  Hopkins 
Hospital  Bulletin,    1898. 
^    F.  P.  Mall  :  "  Development  of  the  Ventral  Abdominal  Walls  in  Man," 
Journ.  of  Morphol,  xiv,  1898. 
J.  P.  McMurrich  :  "The  Phylogeny  of  the  Forearm  Flexors,"  Amer. 
Journ.  of  Anat.,  11,  1903. 
'       J.    P.    McMurrich  :    "  The    Phylogeny    of    the    Palmar    Musculature," 
Amer.  Journ.  of  Anat.,  11,  1903. 
J.   P.   McMurrich:    "The   Phylogeny  of  the  Crural  Flexors,"   Amer. 
Journ.  of  Anat.,  w,  1904. 
-      J.    P.    McMurrich  :    "  The    Phylogeny    of    the    Plantar    Musculature," 
Amer.  Journ.  of  Anat.,  vi,  1907. 

A.  Meek  :  "  Preliminary  Note  on  the  Post-embryonal  History  of 
Striped  Muscle-fibers  in  Mammalia,"  Anat.  Anseigcr,  xiv,  1898. 
(See  also  Anat.  Anzeiger,  xv,  1899.) 

B.  MoRPURGO :  "  Ueber  die  post-embryonale  Entwickelung  der  quer- 
gestreiften  Muskel  von  weissen  Ratten,"  Anat.  Anzeiger,  xv,  1899. 


232  DEVELOPMENT    OF    MUSCULAR    SYSTEM. 

I.  PoPOWSKY :  "  Zur  Entwicklungsgeschichte  des  N.  facialis  beim  Men- 

schen,"  Morphol.  Jahrhuch,  xxiii,  1896. 
\     I.    Popowsky:    "Zur    Entwickelungsgeschichte    der    Dammmuskulatur 

beim  Menschen,"  Anat.  Heftc,  xi,  1899. 
L.  Rethi  :  "  Der  peripheren  Verlauf  der  motorischen  Rachen-  und  Gau- 

mennerven,"  Sitziingsher.  der  kais.  Akad.  JVissensch.  Wicn.  Math- 

Nafunviss.  Classc,  cu,  1893. 
C.    S.    Sherrington:    "Notes    on    the    Arrangement   of    Some    Motor 

Fibers  in  the  Lumbo-sacral  Plexus,"  Jourital  of  Physiol.,  xiu,  1892. 
^       J.   B.    Sutton  :   "  Ligaments,   their   Nature   and   Morphology,"   London, 

1897. 


CHAPTER    IX 

THE    DEVELOPMENT    OF    THE    CIRCULATORY 
AND    LYMPHATIC    SYSTEMS. 

At  present  nothing  is  known  as  to  the  earhest  stages  of 
development  of  the  circulatory  system  in  the  human  embryo, 
but  it  may  be  supposed  that  they  resemble  in  their  funda- 
mental features  what  has  been  observed  in  such  forms  as  the 
rabbit  and  the  chick.  In  both  these  the  system  originates 
in  two  separate  parts,  one  of  which,  located  in  the  embryonic 
mesoderm,  gives  rise  to  the  heart,  while  the  other,  arising 
in  the  extra-embryonic  mesoderm,  forms  the  first  blood- 
vessels. It  will  be  convenient  to  consider  these  two  parts 
separately,  and  the  formation  of  the  blood-vessels  may  be 
first  described. 

In  the  rabbit  the  extension  of  the  mesoderm  from  the 
embryonic  region,  where  it  first  appears,  over  the  yolk-sac 
is  a  gradual  process,  and  it  is  in  the  more  peripheral  por- 
tions of  the  layer  that  the  blood-vessels  first  make  their 
appearance.  They  can  be  distinguished  before  the  splitting 
of  the  mesoderm  has  been  completed,  but  are  always  devel- 
oped in  that  portion  of  the  layer  which  is  most  intimately 
associated  with  the  yolk-sac,  and  consequently  becomes  the 
splanchnic  layer.  The  first  indication  of  the  vessels  is  the 
appearance  in  the  peripheral  portion  of  the  mesoderm  of 
cords  or  minute  patches  of  spherical  cells  (Fig.  124,  A). 
These  increase  in  size  b}^  the  division  and  separation  of  the 
cells  from  one  another  (Fig.  124,  B),  a  clear  fluid  appear- 
ing in  the  intervals  which  separate  them.  Soon  the  cells 
surrounding  each  cord  arrange  themselves  to  form  an  en- 
21  233 


234 


DEVELOPMENT    OF    THE    BLOOD-VESSELS, 


closing  wall,  and  the  cords,  increasing  in  size,  unite  together 
to  form  a  network  of  vessels  in  which  float  the  spherical 
cells  which  may  now  be  known  as  erythrocytes.  Viewed 
from  the  surface  at  this  stage  a  portion  of  the  vascular  area 
of  the  mesoderm  would  have  the  appearance  shown  in  Fig. 
125,  revealing  a  dense  network  of  canals  in  which,  at  inter- 
vals, are  groups  of  erythrocytes  adherent  to  the  walls,  con- 
stitutino-  what  have  been  termed  the  hlood-islands,  while  in 


Fig.    124. — Transverse    Section    through    the   Area    Vasculosa    of 
Rabbit    Embryos    showing    the    Transformation    of    Mesoderm 

CELLS    INTO   THE    VaSCULAR    CoRUS. 

Ec,    Ectoderm;    En,    eiidoderni;    Mc,    mesoderm. — {van    dcr   Sfricht.) 


the  meshes  of  the  network  unaltered  mesoderm  cells  can  be 
seen,  forming  the  so-called  snbstaiice-islauds. 

At  the  periphery  of  the  vascular  area  the  vessels  arrange 
themselves  to  form  a  sums  tenninaUs  enclosing  the  entire 
area,  and  the  vascularization  of  the  splanchnic  mesoderm 
gradually  extends  toward  the  embryo.  Reaching  it,  the 
vessels  penetrate  the  embryonic  tissues  and  eventually  come 
into  connection  with  the  heart,  which  has  already  differen- 
tiated and  has  begun  to  beat  before  the  connection  with  the 
vessels  is  made,  so  that  when  it  is  made  the  circulation  is 


DEVELOPMENT    OF    THE    BLOOD-VESSELS. 


23: 


at  once  established.  Before,  however,  the  vascularization 
reaches  the  embryo  some  of  the  canals  begin  to  enlarge  (Fig. 
126,  A),  producing  arteries  and  veins,  the  rest  of  the  net- 
work forming  capillaries  uniting  these  two  sets  of  vessels, 
and,  this  process  continu- 
ing, there  are  eventually 
differentiated  a  single  orn- 
phalo-mesenteric  {z'itelline ) 
artery  and  two  omphalo- 
mesenteric {vitelline)  veins 
(Fig.   126,  B). 

In  the  human  embryo 
the  small  size  of  the  yolk- 
sac  permits  of  the  exten- 
sion of  the  vascular  area 
over  its  entire  surface  at 
an  early  period,  and  this 
condition  has  already  been 
reached  in  the  earliest 
stages  known  and  conse- 
c[uently  no  sinus  terminalis 
such  as  occurs  in  the  rab- 
bit is  visible.  Otherwise 
the  conditions  are  prob- 
ably   similar    to    what   has    Fig.  125.— Surface  -View  of  a  Por- 

,  1  .,11  .  TION    OF    THE    ArEA    VaSCULOSA    OF 

been   described   above,   the      ^  Chick. 

first     circulation    developed    The  vascular  network  is  represented 
1     •  •    j_    ^        •^^      J_^  bv  the  shaded  portion.     Bi,  Blood- 

bemg    associated    with    the       island;     Si,     substance-island.— 
yolk-sac.  (Dissc.) 

It  is  to  be  noted  that  the  capillary  network  of  the  area 
vasculosa  consists  of  relatively  wide  anastomosing  spaces 
whose  endothelial  lining  rests  directly  upon  the  substance 
islands  (Fig.  125).  In  certain  of  the  embryonic  organs, 
notably  the  liver,  the  metanephros  and  the  heart,  the  net- 


236 


DEXIlLOPMEiSrT    OF    THE    BLOOD-VESSELS. 


work  has  a  similar  character,  consisting  of  wide  anastomos- 
ing spaces  bounded  by  an  endothelium  which  rests  directly, 
or  almost  so,  upon  the  parenchjina  of  the  organ  (the  hepatic 
cyhnders,  the  mesonephric  tubules,  or  the  cardiac  muscle 
trabeculae)  (Figs.  127  and  180,  B).  To  this  form  of  cap- 
illar\-  the  term  sinusoid  has  been  applied  (Alinot),  and  it 
appears  to  be  formed  by  the  expansion  of  the  wall  of  a  pre- 
vioush"  existing  blood-vessel,  which  thus  moulds  itself,  as 
it  were,  over  the  parench^Tna  of  the  organ.     The  true  cap- 


f~- 


_  ,,  .  --    .  A 

Fig.  126. — The  Vasclt-ar  Areas  of  Rabbit  Embryos.  In  B  the  Veins 
ABE  Represented  by  Black  and  the  Network  is  Omitted. — 
{z'aii  Benedcn  and  Julin.) 


illaries,  on  the  other  hand,  are  more  definitel}^  tubular  in 
form,  are  usuall}'^  imbedded  in  mesench3Tnatous  connective 
tissue  and  are  developed  in  the  same  manner  as  the  primar)^ 
capillaries  of  the  area  vasculosa,  by  the  aggregation  of  vasi- 
factive cells  to  form  cords,  and  the  subsequent  hollowing 
out  of  these.  \\  hether  these  vasifactive  cells  are  new  dif- 
ferentiations of  the  embr^^onic  mesenchyme  or  are  budded 


THE    FORMATION    OF    THE    BLOOD.  23/ 

off  from  the  walls  of  existing-  capillaries  which  have  grown 
in  from  extra-embryonic  regions,  is  at  present  undecided. 

The  Formation  of  the  Blood. — The  erythrocytes,  which 
are  the  first  blood-corpuscles,  are  all  nucleated  and  are  for 
a  time  the  only  cells  occurring  in  the  blood,  though  later 
other  cells  arising  in  tissues  exterior  to  the  blood-vessels, 
make  their  way  into  the  vessels,  forming  leukocytes.  From 
their  very  first  formation  then  the  red  (erythrocytes)  and 
white  (leukocytes)  blood-corpuscles  have  a  different  origin, 
and  they  remain  distinct  throughout  life,  one  form  never 
becoming  converted  into  the  other. 

So  long  as  the  formation  of  blood-vessels  is  taking  place 
in  the  extra-embryonic  mesoderm,  so  long  are  new  erythro- 
cytes being  differentiated  from  the  mesoderm.  But  whether 
the  formation  of  blood-vessels  within  the  embryo  results 
from  a  differentiation  of  the  embryonic  mesoderm  in  situ, 
or  from  the  actual  ingrowth  of  vessels  from  the  extra- 
embryonic regions  (His),  is  as  yet  uncertain,  and  hence  it  is 
also  uncertain  whether  erythrocytes  are  differentiated  from 
the  embryonic  mesoderm  or  merely  jDass  into  the  embryonic 
region  from  the  more  peripheral  areas.  However  this  may 
be,  it  is  certain  that  the  erythrocytes  increase  by  division  in 
the  interior  of  the  embryo,  and  that  there  are  certain  por- 
tions of  the  body  in  which  these  divisions  take  place  most 
abundantly,  partly,  perhaps,  on  account  of  the  more  favor- 
able conditions  of  nutrition  which  they  present  and  partly 
because  they  are  regions  where  the  circulation  is  sluggish 
and  permits  the  accumulation  of  erythrocytes.  These  re- 
gions constitute  what  have  been  termed  the  ]i(rinatopoictic 
organs,  and  are  especially  noticeable  in  the  later  stag^es  of 
fetal  life,  diminishing  in  number  and  variety  about  the  time 
of  birth.  It  must  be  remembered,  however,  that  the  life  of 
individual  corpuscles  is  comparatively  short,  their  death  and 
disintegration  taking  place  continually  during  the  entire  life 


238 


THE    FORMATION    OF    THE    BLOOD. 


of  the  individual,  so  that  there  is  a  necessity  for  the  forma- 
tion of  new  corpuscles  and  for  the  existence  of  haemato- 
poietic organs  at  all  stages  of  life. 

In  the  fetus  erythrocytes  in  process  of  division  may  be 
found  in  the  general  circulation  and  even  in  the  heart  itself, 

but  they  are  much  more 
plentiful  in  places  where 
the  blood-pressure  is 
diminished,  as,  for  in- 
stance, in  the  larger 
capillaries  of  the  lower 
limbs  and  in  the  capil- 
laries of  all  the  visceral 
organs  and  of  the  sub- 
cutaneous tissues.  Cer- 
tain organs,  however, 
such  as  the  liver,  the 
spleen,  and  the  bone- 
marrow,  present  especi- 
ally favorable  conditions 
for  the  multiplication 
of  the  blood-cells,  and  in 
these  not  only  are  the 
capillaries  enlarged  so  as  to  afford  resting-places  for  the 
corpuscles,  but  gaps  appear  in  the  walls  of  the  vessels 
through  which  the  blood-elements  may  pass  and  so  come 
into  intimate  relations  with  the  actual  tissues  of  the  organs 
(Fig.  127).  After  birth  the  haematopoietic  function  of  the 
liver  ceases  and  that  of  the  spleen  becomes  limited  to  the 
formation  of  white  corpuscles,  though  the  complete  function 
may  be  re-estaljlished  in  cases  of  extreme  anaemia.  The 
bone-marrow,  however,  retains  the  function  completely, 
being  throughout  life  the  seat  of  formation  of  both  red  and 
white  corpuscles,  the  lymphatic  nodes  and  follicles,  as  well 


Fig.  127. — Section  of  a  Portion  of  the 
Liver  of  a  Rabbit  Embryo  of  5  mm. 

c,  Erythrocytes  in  the  liver  substance 
and  in  a  capillary ;  /;,  hepatic  cells. — ■ 
{van  dcr  Stricht.) 


T-HE    FORMATION    OF    THE    BLOOD.  239 

as  the  spleen,  assisting-  in  the  formation  of  the  latter  elements. 
Until  about  the  second  month  of  development  the  erythro- 
cytes and  leukocytes  are  the  only  elements  found  in  the 
blood,  and  in  the  hsematopoietic  organs  they  may  be  observed 
in  active  mitosis.  In  addition  other  cells,  having  the  same 
general  appearance  as  the  erythrocytes  but  lacking  haemo- 
globin, also  occur,  and  these,  which  may  be  termed  crythro- 
hlasts,  produce  by  division  erythrocytes  in  which  haemo- 
globin gradually  appears.  After  the  second  month,  how- 
ever, a  third  form  of  blood-elements  appears  in  the  form  of 
non-nucleated  discs  containing  haemoglobin,  and  these  may 
be  termed  crytliroplastids.  They  are  derived  from  the  ery- 
throcytes, whose  nuclei,  origi- 
nally reticular  ,  in  structure, 
gradually  condense  to  become 

spherical,    deeplv   staining  t-         „     o  ^ 

^  ^  ■  *    Fig.  128. — Stages  in  the  Trans- 

masses,  and  are  finally  com-      formation  of  an  Erythrocyte 
1,1  i.       1     1      r  ii  INTO  AN  Erythroplastid. —  {van 

pletely     extruded     from     the      ^^^.  stHcht ) 

C3^toplasm    (Fig.    128).     The 

cast-off  nuclei  undergo  degeneration  and  phagocytic  ab- 
sorption by  the  leukocytes,  and  the  masses  of  cytoplasm 
pass  into  the  circulation,  becoming  more  and  more  numerous 
as  development  proceeds,  until  finally  they  are  the  only 
haemoglobin-containing  elements  in  the  blood  and  form 
what  are  properly  termed  the  red  blood-corpuscles.  In  the 
later  fetal  and  the  post-natal  stages  erythrocytes  are  to  be 
found  only  in  the  red  bone-marrow. 

In  the  formation  of  the  new  leukocytes  there  is  a  tendency 
for  the  dividing  cells  to  collect  in  more  or  less  definite  groups 
which  have  been  termed  germ-centers  (Flemming).  The 
new  cells  when  they  first  pass  into  the  circulation  have  a  rela- 
tively large  nucleus  surrounded  by  a  small  amount  of  cyto- 
plasm, and,  since  they  resemble  the  cells  found  in  the  lym- 
phatic vessels,  are  termed  lymphocytes.     In  the  circulation 


240  THE    FORMATION    OF    THE    BLOOD. 

the  nuclei  become  larger  and  the  cytoplasm  more  voluminous 
and  amoeboid,  the  cells  being-  then  known  as  mononuclear 
leukocytes,  and  transitional  forms  lead  from  these  to  still 
larger  cells  with  irregularly  lobed  or  branched  nuclei,  the 
polvinorphonuclcar  leukocytes,  while  these  again  seem  to 
lead  to  polymiclear  cells.  It  is  probable  that  these  various 
kinds  of  cells  stand  in  genetic  relation  to  one  another,  the 
polymorphonuclear  and  polynuclear  forms  perhaps  repre- 
senting the  commencement  of  the  degeneration  and  break- 
ing down  of  the  elements. 

In  the  fetal  haematopoietic  organs  and  in  the  bone-marrow 
of  the  adult  large,  so-called  giant-cells  are  found,  which, 
although  they  do  not  enter  into  the  general  circulation,  are 
yet  associated  with  the  development  of  the  bjood-corpuscles. 
These  giant-cells  as  they  occur  in  the  bone-marrow  are  of 
two  kinds  which  seem  to  be  quite  distinct,  although  both  are 
probably  formed  from  leukocytes.  In  one  kind  the  cyto- 
plasm contains  several  nuclei,  whence  they  have  been  termed 
polycaryocytes,  and  they  seem  to  be  the  cells  which  have  al- 
ready been  mentioned  as  osteoclasts  (p.  165).  In  the  other 
kind  (Fig.  129)  the  nucleus  is  single,  but  it  is  large  and 
irregular  in  shape,  frequently  appearing  as  if  it  were  pro- 
ducing buds.  These  mcgacaryocytes  appear  to  be  phago- 
cytic cells,  having  as  their  function  the  destruction  of  degen- 
erated corpuscles  and  of  the  nuclei  of  the  erythrocytes. 

Little  is  certainly  known  as  yet  as  to  the  origin  of  the 
blood-platelets,  though  the  most  plausible  suggestion  is  that 
they  are  the  fragmented  nuclei  of  broken-down  leukocytes. 

The  question  of  the  origin  of  the  various  forms  of  blood- 
elements  is  a  very  difficult  one,  and  the  opinions  of  some  ob- 
servers are  very  different  from  some  of  the  statements  made 
above.  Thus  it  has  been  maintained  that  the  nuclei  of  the 
erythrocytes  are  not  extruded  in  the  formation  of  erythro- 
plastids,  but  undergo  a  degeneration  within  the  original  cell ; 
that   mesenchyme   cells   of    the   marrow   become   transformed 


THE    FORMATION    OF    THE    HEART. 


241 


into  leukocytes;  that  the  polymorphonuclear  and  polynuclear 
leukocytes  are  not  stages  leading  to  disintegration,  but  repre- 
sent stages  of  amitotic  division,  etc.  It  is  impossible  in  the 
limits  of  the  present  work  to  discuss  these  various  ideas,  and 
the  views  which  have  seemed  to  be  most  strongly  supported 
by  observations  have  been  chosen  for  presentation. 


Fig.  129. — Portion  of  a  Section  from  the  Liver  of  an  Embryo  Cat 

OF   2.7    MM.    showing   A    MegACARYOCYTE    SURROUNDED  BY    ERYTHRO- 
CYTES IN  A  Blood-vessel.- — (Hozvcll.) 

The  Formation  of  the  Heart. — The  heart  makes  its  ap- 
pearance while  the  embryo  is  still  spread  out  upon  the 
surface  of  the  yolk-sac,  and  arises  as  two  separate  portions 
which  only  later  come  into  contact  in  the  median  line. 
On  each  side  of  the  body  near  the  margins  of  the  embry- 
onic area  a  fold  of  the  splanchnopleure  appears,  projecting 
into  the  coelomic  cavity,  and  within  this  fold  a  very  thin- 
walled  sac  is  formed,  probably  by  a  splitting  off  of  its  in- 
nermost cells  (Fig.  130,  A).  Each  fold  will  produce  a 
portion  of  the  muscular  walls  {myocardium)  of  the  heart, 
and  each  sac  part  of  its  endothelium  (endocardium) .  As 
the  constriction  of  the  embryo  from  the  yolk-sac  proceeds, 
the  two  folds  are  gradually  brought  nearer  together  (Fig. 
130,  B),  until  they  meet  in  the  mid-ventral  line,  when  the 
myocardial  folds  and  endocardial    sac  fuse  together   (Fig. 


242 


THE    FORMATION    OF    THE    HEART, 


130,  C)  to  form  a  cylindrical  heart  lying  in  the  mid-ventral 
line  of  the  body,  in  front  of    the  anterior  surface  of    the 


erv 


Fig.    130. — Diagrams    Illustrating    the    Formation    of    the    Heart 

IN  THE  Guinea-pig. 
The  mesoderm  is  represented  in  black  and  the  endocardium  by  a  broken 

line,     am,   Amnion ;   en,  endoderm ;   h,  heart ;   i,  digestive  tract. — 

(After  Strahl  and  Carius.) 


yolk-sac  and  in  what  will  later  be  the  cervical  region  of  the 
body.  At  an  early  stage  the  various  veins  which  have  already 


THE    FORMATION    OF    THE    HEART. 


243 


been  formed,  the  omphalo-mesenterics,  umbilicals,  jugulars 
and  cardinals,  unite  together  to  open  into  a  sac-like  struc- 
ture, the  sinus  venosus,  and  this  opens  into  the  posterior 
end  of  the  heart  cylinder.  The  anterior  end  of  the  cylinder 
tapers  off  to  form  the  aortic  bulb,  which  is  continued  for- 
ward on  the  ventral  surface  of  the  pharyngeal  region  and 
carries  the  blood  away  from  the  heart.     The  blood  accord- 


FiG.  131. — Heart  of  Embryo  of 
2.15  MM.,  FROM  A  Reconstruc- 
tion. 

a,  atrium;  ab,  aortic  bulb;  d,  dia- 
phragm ;  dc,  ductus  Cuvieri ;  /, 
liver;  v,  ventricle;  vj,  jugular 
vein ;  vu,  umbilical  vein. —  (His.) 


Fig.    132. — Heart  of   Embryo  of 

4.2   MM.   SEEN   from   THE  DoRSAL 

Surface. 
DC,     Ductus     Cuvieri ;     lA,     left 
atrium ;    rA,    right    atrium ;    vj, 
jugular  vein;  VI,  left  ventricle; 
vu,  umbilical  vein. —  (His.) 


ingly  opens  into  the  posterior  end  of  the  heart  tube  and 
flows  out  from  its  anterior  end. 

The  simple  cylindrical  form  soon  changes,  however,  the 
heart  tube  in  embryos  of  2.15  mm.  in  length  having  be- 
come bent  upon  itself  into  a  somewhat  S-shaped  curve 
(Fig.  131).  Dorsally  and  to  the  left  is  the  lower  end  into 
which  the  sinus  venosus  opens,  and  from  this  the  heart 
tube  ascends  somewhat  and  then  bends  so  as  to  pass  at  first 


244  THE    FORMATION    OF    THE    HEART. 

ventrally  and  then  downward  and  to  the  right,  where  it 
again  bends  at  first  dorsally  and  then  anteriorly  to  pass  over 
into  the  aortic  bulb.  The  portion  of  the  curve  which  lies 
dorsally  and  to  the  left  is  destined  to  give  rise  to  both  atria, 
the  portion  which  passes  from  rig-ht  to  left  represents  the 
future  left  ventricle,  while  the  succeeding  portion  repre- 
sents the  right  ventricle.  In  later  stages  (Fig.  132)  the 
left  ventricular    portion  drops  downward  in  front  of    the 

atrial  portion,  assuming 
a  more  horizontal  position, 
while  the  portion  which 
represents  the  right  ven- 
tricle is  drawn  forward 
so  as  to  lie  in  the  same 
plane  as  the  left. 

At  the  same  time  two 
small  out-pouchings  de- 
velop from  the  atrial  part 
of    the    heart    and     form 

Fig.  133.-HEART  OF  Embryo  of  5  ^^^^  ^''^^  indications  of  the 
MM.,  Seen  from  in  Front  and  two  atria.  As  develop- 
SLiGHTLY    from    Above. —  (His.)  ,1 

ment  progresses,  these  m- 

crease  in  size  to  form  large  pouches  opening  into  a  common 

atrial  canal   (Fig.   133)  which  is  directly  continuous  with 

the  left  ventricle,  and  as  the  enlargement  of  the  pouches 

continues  their  openings  into  the  canal  enlarge,  until  finally 

the  pouches  become  continuous  with  one  another,  forming 

a  single  large  sac,  and  the  atrial  canal  becomes  reduced  to 

a  short  tube  which  is  slightly  invaginated  into  the  ventricle 

(Fig.  134). 

In  the  meantime  the  sinus  venosus,  which  was  originally 

an  oval  sac  and  opened  into  the  atrial  canal,  has  elongated 

transversely  until    it  has  assumed  the  form  of    a  crescent 

whose  convexity  is  in  contact  with  the  walls  of  the  atria. 


T-HE    FORMATION    OF    THE    HEART. 


245 


and  its  opening  into  the  heart  has  verged  toward  the  right, 
until  it  is  situated  entirely  within  the  area  of  the  right 
atrium.  As  the  enlargement  of  the  atria  continues,  the  right 
horn  and  median  portion  of  the  crescent  are  gradually- 
taken  up  into  their  walls,  so  that  the  various  veins  which 
originally  opened  into  the  sinus  now  open  directly  into  the 
right  atrium  by  a  single  opening,  guarded  by  a  projecting 


\'^V/,./.^//^^^^^\VV 


Fig.   134. — Inner  Surface  of  the  Heart  of  an   Embryo  of   10  mm. 

al,    atrio-ventricular    thickening;    sp,    septum     spurium ;     ss,    septum 

primum;   sv,   septum  ventriculi;   ve,   Eustachian  valve. —  (His.) 

fold  which  is  continued  upon  the  roof  of  the  atrium  as  a  mus- 
cular ridge  known  as  the  septum  spurium  (Fig.  134,  sp). 
The  left  horn  of  the  crescent  is  not  taken  up  into  the  atrial 
wall,  but  remains  upon  its  posterior  surface  as  an  elongated 
sac  forming  the  coronary  sinus. 

The  division  of  the  now  practically  single  atrial  cavity 
into  the  permanent  right  and  left  atria  begins  with  the  for- 
mation of  a  falciform  ridge  running  dorso-ventrally  across 


246 


THE    FORMATION    OF    THE    HEART. 


Si  Sz 


the  roof  of  the  cavity.  This  is  the  atrml  septum  or  septum 
priimiin  (Fig.  134,  ss),  and  it  rapidly  increases  in  size  and 
thickens  upon  its  free  margin,  which  reaches  almost  to  the 
upper  border  of  the  short  atrial  canal    (Fig.    136).     The 

continuity  of  the  two  atria  is  thus 
almost  dissolved,  but  is  soon  re- 
established by  the  formation  in 
the  dorsal  part  of  the  septum  of 
an  opening  which  soon  reaches  a 
considerable  size  and  is  known  as 
the  foramen  ovale  (Fig.  133,  fo). 
Close  to  the  atrial  septum,  and 
parallel  with  it,  a  second  ridge 
appears  in  the  roof  and  ventral 


Fig.    135. — Heart    of    Em- 
bryo   OF     10.2     CM.     FROM 

wiiicH  Half  of  the 
Right  Auricle  Has  Been 
Removed. 
fo,  Foramen  ovale;  pa,  pul- 
monary artery ;  Si,  septum 
primum ;  S2,  septum  secun- 
dum ;  Sa,  systemic  aorta ; 
V,  right  ventricle;  vci 
and  vcs,  inferior  and 
superior  venje  cavse;  Ve, 
Eustachian  valve. 


wall  of  the  right  atrium.     This 


septum  secundum  {S2)  is  from 
the  beginning  very  much  thicker 
than  the  atrial  septum,  and  its 
free  edge,  arching  around  the 
ventral  edge  and  floor  of  the  fora- 
men ovale,  becomes  continuous 
with  the  left  lip  of  the  fold  which 
guards  the  opening  of  the  sinus 
venosus  and  with  this  forms  the  annulus  of  Vieiissens  of 
the  adult  heart. 

When  the  absorption  of  the  sinus  venosus  into  the  wall  of 
the  right  atrium  has  proceeded  so  far  that  the  veins  com- 
municate directly  with  the  atrium,  the  vena  cava  superior 
opens  into  it  at  the  upper  part  of  the  dorsal  wall,  the  vena 
cava  inferior  more  laterally,  and  below  this  is  the  smaller 
opening  of  the  coronary  sinus.  The  upper  portion  of  the 
right  lip  of  the  fold  which  originally  surrounded  the  open- 
ing of  the  sinus  venosus,  together  with  the  septum  spurium, 
gradually  disappears  ;  the  lower  portion  persists,  however, 


THE    FORMATION    OF    THE    HEART.  247 

and  forms  (i)  the  Eustacliiaii  valve  (Fig.  135,  Ve),  guard- 
ing the  opening  of  the  inferior  cava  and  directing  the  blood 
entering  by  it  toward  the  foramen  ovale,  and  (2)  the 
Thebesian  valve,  which  guards  the  opening  of  the  coronary 
sinus.  At  first  no  veins  communicate  with  the  left  atrium, 
but  on  the  development  of  the  lungs  and  the  establishment 
of  their  vessels,  the  pulmonaiy  veins  make  connection  with 
it.  TwO'  veins  arise  from  each  lung,  and  as  they  pass 
toward  the  heart  they  unite  in  pairs,,  the  two  vessels  so 
formed  again  uniting  to  form  a  single  short  trunk  which 
opens  into  the  upper  part  of  the  atrium  (Fig.  136,  Vep). 
As  is  the  case  with  the  right  atrium  and  the  sinus  venosus, 
the  expansion  of  the  left  atrium  brings  about  the  absorp- 
tion of  the  short  single  trunk  into  its  walls,  and,  the  ex- 
pansion continuing,  the  two  vessels  are  also  absorbed,  so 
that  eventually  the  four  primary  veins  open  independently 
into  the  atrium. 

While  the  atrial  septa  have  been  developing  there  has 
appeared  on  the  dorsal  wall  of  the  atrial  canal  a  tubercle- 
like  thickening  of  the  endocardium,  and  a  similar  thicken- 
ing also  forms  on  the  ventral  wall.  These  endocardial 
cushions  increase  in  size  and  finally  unite  together  by  their 
tips,  forming  a  complete  partition,  dividing  the  atrial  canal 
into  a  right  and  left  half  (Fig.  136).  AA^ith  the  upper 
edge  of  this  partition  the  thickened  lower  edge  of  the  atrial 
septum  unites,  so  that  the  separation  of  the  atria  would  be 
complete  were  it  not  for  the  foramen  ovale. 

While  these  changes  have  been  taking  place  in  the  atrial 
portion  of  the  heart,  the  separation  of  the  right  and  left 
ventricles  has  also  been  progressing,  and  in  this  two  dis- 
tinct septa  take  part.  From  the  floor  of  the  ventricular 
cavity  along  the  line  of  junction  of  the  right  and  left  por- 
tions a  ridge,  composed  largely  of  muscular  tissue,  arises 
(Figs.  134  and  136),  and,  growing    more    rapidly    in    its 


248 


THE    FORMATION    OF    THE    HEART. 


dorsal  than  its  ventral  portion,  it  comes  into  contact  and 
fuses  with  the  dorsal  part  of  the  partition  of  the  atrial 
canal.     Ventrally,  however,  the  ridge,  known  as  the  ven- 


SM 


En.r 


En.s 


Fig.  136. — Section  through  a  Reconstruction  of  the  Heart  of  a 
Rabbit  Embryo  of  io.i  mm. 

Ad  and  Adx,  Right  and  As,  left  atrium ;  Brv-i.  and  Bvoi,  lower  ends  of 
the  ridges  which  divide  the  aortic  bulb ;  Rn,  endocardial  cushion ; 
En.r  and  En.s,  thickenings  of  the  cushion ;  la,  interatrial  and 
Iv,  interventricular  communication ;  S\,  septum  primum ;  Sd,  right 
and  Ss,  left  horn  of  the  sinus  venosus ;  S.iv,  ventricular  septum ; 
SM,  opening  of  the  sinus  venosus  into  the  atrium ;  Vd,  right  and 
Vs,  left  ventricle;  Vej,  jugular  vein;  Vep,  pulmonary  vein;  Vvd 
and  Vvs,  right  and  left  limbs  of  the  valve  guarding  the  opening 
of  the  sinus  venosus. —  (Born.) 

iricitlar  septum,  fails  to  reach  the  ventral  part  of  the  par- 
tition, so  that  an  oval  foramen,  situated  just  helow  the 
point  where  the  aortic  bulb  arises,   still  remains  between 


THE    FORMATION    OF    THE    HEART.  249 

the  two  ventricles.  This  opening  is  finally  closed  by  what  is 
termed  the  aortic  septum.  This  makes  its  appearance  in  the 
aortic  bulb  just  at  the  point  where  the  first  lateral  branches 
which  give  origin  to  the  pulmonary  arteries  (see  p.  256) 
arise,  and  is  formed  by  the  fusion  of  the  free  edges  of  two 
ridges  which  develop  on  opposite  sides  of  the  bulb.  From 
its  point  of  origin  it  gradually  extends  down  the  bulb  until 
it  reaches  the  ventricle,  where  it  fuses  with  the  free  edge 
of  the  ventricular  septum  and  so  completes  the  separation 
of  the  two  ventricles  (Fig.  135).  The  bulb  now  consists 
of  two  vessels  lying  side  by  side,  and  owing  to  the  position 
of  the  partition  at  its  anterior  end,  one  of  these  vessels, 
that  which  opens  into  the  right  ventricle,  is  continuous  with 
the  pulmonary  arteries,  while  the  other,  which  opens  into 
the  left  ventricle,  is  continuous  with  the  rest  of  the  vessels 
which  arise  from  the  forward  continuation  of  the  bulb.  As 
soon  as  the  development  of  the  partition  is  completed,  two 
grooves,  corresponding  in  position  to  the  lines  of  attach- 
ment of  the  partition  on  the  inside  of  the  bulb,  make  their 
appearance  on  the  outside  and  gradually  deepen  until  they 
finally  meet  and  divide  the  bulb  into  two  separate  vessels, 
one  of  which  is  the  pulmonary  aorta  and  the  other  the  sys- 
temic aorta. 

In  the  early  stages  of  the  heart's  development  the  muscle 
bundles  which  compose  the  wall  of  the  ventricle  are  very 
loosely  arranged,  so  that  the  ventricle  is  a  somewhat  spongy 
mass  of  muscular  tissue  with  a  relatively  small  cavity.  As 
development  proceeds  the  bundles  nearest  the  outer  surface 
come  closer  together  and  form  a  compact  layer,  those  on 
the  inner  surface,  however,  retaining  their  loose  arrange- 
ment for  a  longer  time  (Fig.  136).  The  lower  edge  of 
the  atrial  canal  becomes  prolonged  on  the  left  side  into  one, 
and  on  the  right  side  into  two,  flaps  which  project  down- 
ward into  the  ventricular  cavity,  and    an  additional  flap 


250 


THE    FORMATION    OF    THE    HEART. 


arises  on  each  side  from  the  lower  edge  of  the  partition 
of  the  atrial  canal,  so  that  three  flaps  occur  in  the  right 
atrio-ventricular  opening  and  two  in  the  left.     To  the  under 


Oi—j 


'Fav.d 


S.ivr 


Fig.  137. — Diagrams  of  Sections  through  the  Heart  of  Embryo 
Rabbits  to  show  the  Mode  of  Division  of  the  Ventricles  and 
OF  THE  Atrio-ventricular  Orifice. 

Ao,  Aorta;  Ar.p,  pulmonary  artery;  B,  aortic  bulb;  Bzvz,  one  of  the 
ridges  which  divide  the  bulb ;  Eo,  and  Eu,  upper  and  lower  thicken- 
ings of  the  margins  of  the  atrio-ventricular  orifice;  F.av.c,  the 
original  atrio-ventricular  orifice;  F.av.d  and  F.av.s,  right  and 
left  atrio-ventricular  orifices ;  Oi,  interventricular  communication ; 
S.iv,  ventricular  septum;  Vd  and  Vs,  right  and  left  ventricles. 


THE    FORMATION    OF    THE    HEART. 


251 


surfaces  of  these  flaps  the  loosely  arranged  muscular  tra- 
beculcC  of  the  ventricle  are  attached,  and  muscular  tissue 
also  occurs  in  the  flaps.  This  condition  is  transitory,  how- 
ever ;  the  muscular  tissue  of  the  flaps  degenerates  to  form 
a  dense  layer  of  connective  tissue,  and  at  the  same  time  the 
muscular  trabeculse  undergo  a  condensation.  Some  of 
them  separate  from  the  flaps,  which  represent  the  atrio- 
ventricular valves,  and  form  muscle  bundles  which  may 
fuse  throughout  their  entire  length  with  the  more  compact 
portions  of  the  ventricular  walls,  or  else  may  be  attached 


Fig.   138.— Diagrams   showing  the  Development  of  the  Auriculo- 

VENTRICULAR    VaLVES. 

b,  Muscular  trabeculas;  cht,  chordae  tendinese;  mk  and  mk^,  valve;  pm. 
musculus  papillaris;  tc,  trabeculse  carnese;  v,  ventricle.- — {From 
Hertwig,  after  Gegenbaur.) 


only  by  their  ends,  forming  loops;  these  two  varieties  of 
muscle  bundles  constitute  the  trabecule  cameos  of  the  adult 
heart.  Other  bundles  may  retain  a  transverse  direction, 
passing  across  the  ventricular  cavity  and  forming  the  so- 
called  moderator  hands;  while  others,  again,  retaining 
their  attachment  to  the  valves,  condense  only  at  their  lower 
ends  to  form  the  musculi  papillares,  their  upper  portions 
undergoing  conversion  into  strong  though  slender  fibrous 
cords,  the  chordce  tendinece  (Fig.  138). 

The  endocardinal  lining  of    the  ventricles  is  at  first  a 
simple  sac  separated  by  a  distinct  interval  from  the  myocar- 


252 


THE    FORMATION    OF    THE    HEART. 


Fig.  139. — Diagrams  Il- 
lustrating THE  For- 
mation OF  THE  Semi- 
lunar Valves. —  {Ge- 
genbaur.) 


diiim,  but  when  the  condensation  of  the  muscle  trabecul?e 
occurs  the  endocardium  appHes  itself  closely  to  the  irregu- 
lar surface  so  formed,  dipping  into  all  the  crevices  between 
the  trabeculae  carnese  and  wrapping  itself  around  the  mus- 
culi  papillares  and  chordae  tendinese  so  as  to  form  a  com- 
plete lining  of  the  inner  surface  of  the  myocardium. 

The  aortic  and  pulmonary  semilunar  valves  make  their 
appearance,  before  the  aortic  bulb 
undergoes  its  longitudinal  split- 
ting, as  four  tubercle-like  thicken- 
ings of  connective  tissue  situated 
on  the  inner  wall  of  the  bulb  just 
where  it  arises  from  the  ventricle. 
When  the  division  of  the  bulb  occurs, 
two  of  the  thickenings,  situated  on 
opposite  sides,  are  divided,  so  that 
both  the  pulmonary  and  systemic 
aortse  receive  three  thickenings  (Fig.  139).  Later  the 
thickening  becomes  hollowed  out  on  the  surfaces  directed 
away  from  the  ventricles  and  are  so  converted  into  the 
pouch-like  valves  of  the  adult. 

Changes  in  the  Heart  after  Birth. — The  heart  when  first 
formed  lies  far  forward  in  the  neck  region  of  the  embryo, 
between  the  head  and  the  anterior  surface  of  the  yolk-sac, 
and  from  this  position  it  gradually  recedes  until  it  reaches 
its  final  position  in  the  thorax.  And  not  only  does  it  thus 
change  its  relative  position,  but  the  direction  of  its  axes  also 
changes.  For  at  an  early  stage  the  ventricles  lie  directly 
in  front  of  (i.  e.,  ventrad  to)  the  atria  and  not  below  them 
as  in  the  adult  heart,  and  this  primitive  condition  is  retained 
until  the  diaphragm  has  reached  its  final  position  (see 
p.  342). 

In  addition  to  these  changes  in  position,  important 
changes    also    occur    in    the    atrial    septum    after    birth. 


DEVELOPMENT    OF    THE    ARTERIAL    SYSTEM.  253 

Throughout  the  entire  period  of  fetal  hfe  the  foramen 
ovale  persists,  permitting  the  blood  returning  from  the 
placenta  and  entering  the  right  atrium  to  pass  directly 
across  the  left  atrium,  thence  to  the  left  ventricle,  and  so 
out  to  the  body  through  the  systemic  aorta  (see  p.  284). 
At  birth  the  lungs  begin  to  function  and  the  placental  cir- 
culation is  cut  off,  so  that  the  right  atrium  receives  only 
venous  blood  and  the  left  only  arterial;  a  persistence  of 
the  foramen  ovale  beyond  this  period  would  be  injurious, 
since  it  would  permit  of  a  mixture  of  the  arterial  and  venous 
bloods,  and,  consequently,  it  closes  completely  soon  after 
birth.  The  closure  is  made  possible  by  the  fact  that  during 
the  growth  of  the  heart  in  size  the  portion  of  the  atrial 
septum  which  is  between  the  edge  of  the  foramen  ovale 
and  the  dorsal  wall  of  the  atrium  increases  in  width,  so 
that  the  foramen  is  carried  further  and  further  away  from 
the  dorsal  wall  of  the  atrium  and  comes  to  be  almost  com- 
pletely overlapped  by  the  annulus  of  Vieussens  (Fig.  133). 
This  process  continuing,  the  dorsal  portion  of  the  atrial 
septum  finally  overlaps  the  free  edge  of  the  annulus,  and 
after  birth  the  fusion  of  the  overlapping  surfaces  takes 
place  and  the  foramen  is  completely  closed. 

In  a  large  percentage  (25  to  30  per  cent.)  of  individuals  the 
fusion  of  the  surfaces  of  the  septum  and  annulus  is  not  com- 
plete, so  that  a  slit-like  opening  persists  between  the  two  atria. 
This,  however,  does  not  allow  of  any  mingling  of  the  blood  in 
the  two  cavities,  since  when  the  atria  contract  the  pressure 
of  the  blood  on  both  sides  will  force  the  overlapping  folds  to- 
gether and  so  practically  close  the  opening.  Occasionally  the 
growth  of  the  dorsal  portion  of  the  septum  is  imperfect  or  is 
inhibited,  in  which  case  closure  of  the  foramen  ovale  is  im- 
possible. 

The  Development  of  the  Arterial  System. — It  has  been 
seen  that  the  formation  of  the  blood-vessels  begins  in  the 
extra-embryonic  splanchnic  mesoderm  surrounding  the  yolk- 
sac  and  extends  thence  toward  the  embryo.     The  two  orig- 


254 


DEVELOPMENT    OF    THE    ARTERIAL    SYSTEM. 


inal  omphalo-mesenteric  arteries,  entering  the  body  of  the 
embryo  along  the  yolk-stalk,  make  their  way  to  the  dorsal 
wall  of  the  abdomen,  and,  growing  forward  and  backward, 
give  rise  to  two  longitudinal  stems,  the  representatives  of 


Fig.  140. — Reconstruction  of  Embryo  of  2.6  mm. 
am,  Amnion;  B,  belly-stalk;  E,  optic  evagination ;  H,  heart;  Mn,  man- 
dibular process ;  O,  auditory  capsule ;  om,  omphalo-mesenteric  vein ; 
V,   umbilical   vein;    Y,  yolk-stalk. —  (His.) 


the  dorsal  aorta.  From  near  the  posterior  ends  of  these 
there  arise  at  an  early  stage  two  branches,  which  pass  out 
along  the  allantois  into  the  belly-stalk  and  so  to  the  chorionic 


DEVELOPMENT    OF    THE    ARTERIAL    SYSTEM. 


255 


villi,  forming  the  allantoidean  or  umbilical  arteries,  while 

anteriorly  each  aorta  sends  branches  ventrally  in  the  anterior 

branchial  arches,  and  these,  uniting  together,  pass  backward 

along  the  floor  of  the  pharynx  to  become  continuous  with  the 

aortic  bulb  (Fig.  140).     Later  the  two  dorsal  aortse  fuse 

together  as   far   forward 

as  the  region  of  the  eighth 

cervical  segment  to  form 

a  single  trunk    (Fig. 

141),    and    the   left    om- 

phalo-mesenteric    artery 

disappears,  the  right  one 

persisting    to     form    the 

superior  mesenteric  artery 

of  the  adult. 

It  will  be  convenient  to 
consider  first  the  history 
of  the  vessels  which  pass 
ventrally  in  the  branchial 
arches.  Altogether,  six 
of  these  vessels  are  de- 
veloped, the  fifth  being 
rudimentary  and  transi- 
tory, and  when  fully 
formed  they  have  an  ar- 
rangement which  may  be 
understood  from  the 
diagram  (Fig.  141).  This  arrangement  represents  a  condi- 
tion which  is  permanent  in  the  lower  vertebrates.  In  the 
fishes  the  respiration  is  performed  by  means  of  gills  devel- 
oped upon  the  branchial  arches,  and  the  heart  is  an  organ 
which  receives  venous  blood  from  the  body  and  pumps  it  to 
the  gills,  in  which  it  becomes  arterialized  and  is  then  collected 
into  the  dorsal  aortas,  which  distribute  it  to  the  body.     But 


Fig. 

THE 


a' 

141. — Diagram  Illustrating 
Primary  Arrangement  of 
THE  Branchial  Arch  Vessels. 
a,  aorta ;  ah,  aortic  bulb ;  ec,  external 
carotid ;  ic,  internal  carotid ;  sc, 
subclavian ;  I-VI,  branchial  arch 
vessels. 


256  DEVELOPMENT    OF    THE    ARTERIAL    SYSTEM. 

in  terrestrial  animals,  with  the  loss  of  the  gills  and  the  devel- 
opment of  the  lungs  as  respiratory  organs,  the  capillaries  of 
the  gills  disappear  and  the  afferent  and  efferent  branchial 
vessels  become  continuous,  the  condition  represented  in  the 
diagram  resulting. 

But  this  condition  is  merely  temporary  in  the  mammalia 
and  numerous  changes  occur  in  the  arrangement  of  the 
vessels  before  the  adult  plan  is  realized.  The  first  change 
is  a  disappearance  of  the  vessel  of  the  first  arch,  the  ventral 
stem  from  which  it  arose  being  continued  forward  to  form 
the  temporal  arteries,  giving  off  near  the  point  where  the 
branchial  vessel  originally  arose  a  branch  which  represents 
the  internal  maxillary  artery  in  part,  and  possibly  also  a  sec- 
ond branch  which  represents  the  external  maxillary  (His). 
A  little  later  the  second  branchial  vessel  also  degenerates 
(Fig.  142),  a  branch  arising  from  the  ventral  trunk  near 
its  former  origin,  possibly  representing  the  future  lingual 
artery  (His),  and  then  the  portion  of  the  dorsal  trunk 
which  intervenes  between  the  third  and  fourth  branchial 
vessels  vanishes,  so  that  the  dorsal  trunk  anterior  to  the 
third  branchial  arch  is  cut  off  from  its  connection  with  the 
dorsal  aorta  and  forms,  together  with  the  vessel  of  the  third 
arch,  the  internal  carotid,  while  the  ventral  trunk,  anterior 
to  the  point  of  origin  of  the  third  vessel,  becomes  the  exter- 
nal carotid,  and  the  portion  which  intervenes  between  the 
third  and  fourth  vessels  becomes  the  common  carotid  (Fig. 

143)- 

The  rudimentary  fifth  vessel,  like  the  first  and  second, 
disappears,  but  the  fourth  persists  to  form  the  aortic  arch, 
there  being  at  this  stage  of  development  two  complete  aortic 
arches.  From  the  sixth  vessel  a  branch  arises  which  passes 
backward  to  the  lungs,  forming  the  pulmonary  artery,  and 
the  portion  of  the  vessel  of  the  right  side  which  intervenes 
between  this  and  the  aortic  arch  disappears,  while  the  corre- 


DEVELOPMENT    OF    THE    ARTERIAL    SYSTEM. 


257 


sponding  portion  of  the  left  side  persists  until  after  birth, 
iorm'mg  the  ductus  arteriosus  {ductus  Botalli)  (Fig.  143). 
When  the  longitudinal  division  of  the  aortic  bulb  occurs 
(p.  249),  the  septum  is  so  arranged  as  to  place  the  sixth 
arch  in  communication  with  the  right  ventricle  and  the 
remaining  vessels  in  connection  with  the  left  ventricle,  the 
only  direct  communication  between  the  systemic  and  pul- 


FiG.  142. — Arterial  System  of  an  Embryo  of  10  mm. 

Ic,  Internal  carotid;  P,  pulmonary  artery;  Ve,  vertebral  artery;  ///  to 

VI,  persistent  branchial  vessels. —  (His.) 

monary  vessels  being  by  way  of  the  ductus  arteriosus,  whose 
significance  will  be  explained  later  (p.  284). 

One  other  change  is  still  necessary  before  the  vessels 
acquire  the  arrangement  which  they  possess  during  fetal 
life,  and  this  consists  in  the  disappearance  of  the  lower  por- 
tion of  the  right  aortic  arch  (Fig.  143),  so  that  the  left 
arch  alone  forms  the  connection  between  the  heart  and  the 
dorsal  aorta.  The  upper  part  of  the  right  aortic  arch  per- 
sists to  form  the  proximal  part  of  the  right  subclavian 
artery,  the  portion  of  the  ventral  trunk  which  unites  the 
arch  with  the  aortic  bulb  becoming  the  innominate  artery. 
23 


258 


DEVELOPMENT    OF    THE   ARTERIAL    SYSTEM. 


t^C 


From  the  entire  length  of  the  thoracic  aorta,  and  in  the 
embryo  from  the  aortic  arches,  lateral  branches  arise  corre- 
sponding to  each  seg- 
ment and  accompany- 
ing the  segmental 
nerves.  The  first  of 
these  branches  arises 
just  below  the  point 
of  union  of  the  vessel 
of  the  sixth  arch  with 
the  dorsal  trunk  and 
accompanies  the  hypo- 
glossal nerve  (Fig.  144, 
/z),  and  that  which  ac- 
companies the  seventh 
cervical  nerve  arises 
just  above  the  point  of 
union  of  the  two  aortic 
arches  (Fig.  144,  s), 
and  extends  out  into 
the  limb  bud,  forming 
the  subclavian  artery.* 
Further  down  twelve 
pairs  of  lateral  branches, 


Fig.  143. — Diagram  Illustrating  the 
Changes  in  the  Branchial  Arch 
Vessels. 

a,  aorta ;  da,  ductus  arteriosus ;  ec,  ex- 
ternal carotid;  %c,  internal  carotid;  pa, 
pulmonary  artery;  sc,  subclavian; 
I-VI,  aortic  arch  vessels. 


arising   from   the  tho- 


racic portion  of  the 
aorta,  represent  the  in- 
tercostal arteries,  and 
still  lower  four  pairs  of 
lumbar  arteries  are 
formed,  the  fifth  lumljars  being  represented  by  two  large 
branches,  the  common  iliacs,  which  seem  from  their  size  to 


*  It  must  be  remembered  that  the  right  subclavian  of  the  adult  is 
more  than  equivalent  to  the  left,  since  it  represents  the  fourth  branchial 
vessel  -\-a  portion  of  the  dorsal  longitudinal  trunk -f  the  lateral  seg- 
mental branch    (see  Fig.  142). 


DEVELOPMENT    OF    THE   ARTERIAL   SYSTEM. 


2-59 


be  the  continuations  of  the  aorta  rather  than  branches  of  it. 
The  true  continuation  of  the  aorta  is,  however,  the  middle 
sacral  artery,  which  represents  in  a  degenerated  form  the 
caudal  prolongation  of 
the  aorta  of  other  mam- 
mals, and,  like  this, 
gives  off  lateral  branches 
corresponding  to  the 
sacral  segments. 

In  addition  to  the  seg- 
mental lateral  branches 
arising  from  the  aorta, 
visceral  branches,  which 
have  their  origin  rather 
from  the  ventral  sur- 
face, also  occur.  In  em- 
bryos of  5  mm.  these 
branches  are  arranged 
in  a  segmental  manner, 
a  median  unpaired  vessel 
passing  to  the  digestive 
tract  and  a  pair  of  more 
lateral  branches  passing 
to  the  mesonephros 
(see  p.  363)  correspond- 
ing to  each  of  the  paired 


ICo.    JM 


Fig.  144. — Diagram  showing  the  Re- 
lations OF  THE  Lateral  Branches 
TO  THE  Aortic  Arches. 

EC,  External  carotid ;  h,  lateral  branch 
accompanying  the  hypoglossal  nerve ; 
IC ,  internal  carotid ;  ICo,  intercostal ; 
IM,  internal  mammary ;  s,  sub- 
clavian ;  V,  vertebral ;  /  to  Vlll, 
lateral  cervical  branches ;  i,  2,  lateral 
thoracic  branches. 


branches  passing  to  the 
body  wall.  As  development  proceeds  the  great  majority 
of  these  visceral  branches  disappear,  certain  of  the  lateral 
ones  persisting,  however,  to  form  the  renal,  internal  sper- 
matic, and  hypogastric  arteries  of  the  adult,  while  the 
unpaired  branches  are  represented  only  by  the  coeliac 
artery  and  the  superior  and  inferior  mesenteries.  The 
superior  mesenteric   artery  is  the  adult  representative  of 


266  DEVELOPMENT    OF    THE    ARTERIAL    SYSTEM. 

the  omphalo-mesenteric  artery  of  the  embryo  and  arises 
from  the  aorta  by  two,  three  or  more  roots,  which  corre- 
spond to  the  fifth,  fourth  and  higher  thoracic  segments. 
Later,  all  but  the  lowest  of  the  roots  disappear  and  the  per- 
sisting one  undergoes  a  downward  migration  in  accordance 
with  the  recession  of  the  diaphragm  and  viscera  (see  p. 
342),  until  in  embryos  of  17  mm.  it  lies  opposite  the  first 
lumbar  segment.  Similarly  the  coeliac  and  inferior  mesen- 
teric arteries,  which  when  first  recognizable  in  embryos  of 
9  mm.  correspond  with  the  fourth  and  twelfth  thoracic 
segments  respectively,  also  undergo  a  secondary  downward 
migration,  the  coeliac  artery  in  embryos  of  17  mm.  arising 
opposite  the  twelfth  thoracic  and  the  inferior  mesenteric 
opposite  the  third  lumbar  segment. 

One  of  the  pairs  of  visceral  branches  becomes  especially 
enlarged  to  form  the  umbilical  arteries  of  the  embryo.  It 
seems  probable  that,  like  the  omphalo-mesenterics,  each  of 
these  arteries  primarily  arises  from  the  aorta  by  several 
segmental  roots,  but  in  the  earliest  stage  in  which  it  has 
been  observed  it  arises  by  a  single  root  from  the  third  lum- 
bar segment  (Fig.  145,  U').  Each  artery  passes  forward 
along  the  sides  of  the  intestine,  ventral  to  the  Wolffian  duct 
(see  p.  361),  and  is  thence  continued  out  along  the  allantois 
to  the  chorionic  villi.  Later  this  original  stem  is  joined, 
not  far  from  its  origin,  by  what  appears  to  be  the  lateral 
somatic  branch  of  the  fifth  lumbar  segment,  whereupon  the 
proximal  part  of  the  original  umbilical  vessel  degenerates 
and  the  umbilical  comes  to  arise  from  the  somatic  branch, 
which  is  the  common  iliac  artery  of  adult  anatomy  (Fig. 
145).  Hence  it  is  that  this  vessel  in  the  adult  gives  origin 
both  to  branches  such  as  the  external  iliac,  the  gluteal,  the 
sciatic  and  the  internal  pudendal,  which  are  distributed  to 
the  body  walls  or  their  derivatives,  and  to  others,  such  as 


DEVELOPMENT    OF    THE   ARTERIAL    SYSTEM. 


261 


the  vesical,  inferior  hsemorrhoidal  and  uterine,  which  are 
distributed  to  the  pelvic  viscera.  At  birth  the  portions  of 
the  umbilical  arteries  beyond  the  umbilicus  are  severed  when 
the  umbilical  cord  is  cut,  and  their  intra-embryonic  portions, 
which  have  been  called  the  hypogastric  arteries,  quickly 
undergo  a  reduction  in  size.  Their  proximal  portions  re- 
main functional  as  the 
superior  vesical  arteries, 
carrying  blood  to  the 
urinary  bladder,  but  the 
portions  which  intervene 
between  the  bladder  and 
the  umbilicus  become  re- 
duced to  solid  cords, 
forming  the  obliterated 
hypogastric  arteries  of 
adult  anatomy. 

In  its  general  plan, 
accordingly,  the  arterial 
system  may  be  regarded 

as  consisting  of  a  pair   „  ^  ,- 

^_  ^         Fig.    145. — Diagram    Illustrating    thk 

of  longitudinal  vessels 
which  fuse  together 
throughout  the  greater 
portion  of  their  length  to 
form  the  dorsal  aorta, 
from  which  there  arise 
segmentally  arranged  lateral  somatic  branches  and  ventral 
and  lateral  visceral  branches.  With  the  exception  of  the 
aortic  trunks  (together  with  their  anterior  continuations,  the 
internal  carotids)  and  the  external  carotids,  no  longitudinal 
arteries  exist  primarily.  In  the  adult,  however,  several 
longitudinal  vessels,  such  as  the  vertebrals,  internal  mam- 
mary,  and   epigastric   arteries,    exist.     The    formation    of 


Development  of  the  Umbilical  Ar- 
teries. 
A,  Aorta ;  CIl,  common  iliac ;  E.II,  ex- 
ternal iliac ;  G,  gluteal ;  ///,  internal 
iliac;  IP,  internal  pudic;  IV,  inferior 
vesical ;  Sc,  sciatic ;  U,  umbilical ;  U'. 
primary  proximal  portion  of  the  um- 
bilical; wd,  Wolfifian  duct. 


262  DEVELOPMENT    OF    THE    ARTERIAL   SYSTEM. 

these  secondary  longitudinal  trunks  is  the  result  of  a  devel- 
opment between  adjacent  vessels  of  anastomoses,  which 
become  larger  and  more  important  blood-channels  than  the 
original  vessels. 

At  an  early  stage  each  of  the  lateral  branches  of  the 
dorsal  aorta  gives  off  a  twig  which  passes  forward  to  anas- 
tomose with  a  backwardly  directed  twig  from  the  next 
anterior  lateral  branch,  so  as  to  form  a  longitudinal  chain 
of  anastomoses  along  each  side  of  the  neck.  In  the  earliest 
stage  at  present  known  the  chain  starts  from  the  lateral 
branch  corresponding  to  the  first  cervical  (suboccipital)  seg- 
ment and  extends  forward  into  the  skull  through  the  fora- 
men magnum,  terminating  by  anastomosing  with  the  internal 
carotid.  To  this  original  chain  other  links  are  added  from 
each  of  the  succeeding  cervical  lateral  branches  as  far  back 
as  the  seventh  (Figs.  146  and  144).  But  in  the  meantime 
the  recession  of  the  heart  toward  the  thorax  has  begun,  with 
the  result  that  the  common  carotid  stems  are  elongated  and 
the  aortic  arches  are  apparently  shortened  so  that  the  sub- 
clavian arises  on  the  left  side  almost  opposite  the  point 
where  the  aorta  was  joined  by  the  sixth  branchial  vessel 
As  this  apparent  shortening  proceeds,  the  various  lateral 
branches  which  give  rise  to  the  chain  of  anastomoses,  with 
the  exception  of  the  seventh,  disappear  in  their  proximal 
portions  and  the  chain  becomes  an  independent  stem,  the 
vertebral  artery,  arising  from  the  seventh  lateral  branch, 
which  is  the  subclavian. 

The  recession  of  the  heart  is  continued  until  it  lies  below 
the  level  of  the  upper  intercostal  arteries,  and  the  upper 
two  of  these,  together  with  the  last  cervical  branch  on  each 
side,  lose  their  connection  with  the  dorsal  aorta,  and,  send- 
ing off  anteriorly  and  posteriorly  anastomosing  twigs,  de- 
velop a  short  longitudinal  stem,  the  superior  intercostal, 
which  opens  into  tlie  subclavian. 


DEVELOPMENT    OF   THE   ARTERIAL   SYSTEM. 


263 


The  intercostals  and  their  abdominal  representatives,  the 
lumbars  and  iHacs,  also  give  rise  to  longitudinal  anastomos- 
ing twigs  near  their  ventral  ends  (Fig.  147),  and  these 
increasing  in  size  give  rise  to  the  internal  mammary  and 


A.V.CV. 


Fig.  146. — The  Development  of  the  Vertebral  Artery  in  a  Rabbit 
Embryo  of  Twelve  Days. 

IIIA.B  to  VIA.B,  Branchial  arch  vessels ;  Ap,  pulmonary  artery. 
A.v.c.b  and  A.v.cv,  cephalic  and  cervical  portions  of  the  vertebral 
artery;  A.s,  subclavian;  C.d  and  C.v  internal  and  external  carotid; 
ISp.G,  spinal  ganglion. —  {Hochstetter.) 

inferior  epigastric  arteries,  which  together  form  continuous 
stems  extending  from  the  subclavians  to  the  external  iliacs 


204  DEVELOPMENT    OF    ARTERIES    OF    LIMBS.'^ 

in  the  ventral  abdominal  walls.  The  superficial  epigastrics 
and  other  secondary  longitudinal  vessels  are  formed  in  a 
similar  manner. 

The  Development  of  the  Arteries  of  the  Limbs. — 
Much  information  is  still  required  before  the  complete  his- 
tory of  the  development  of  the  arteries  of  the  limbs  can  be 
written,  and  at  present  one  must  rely  largely  upon  the  facts 


Fig.  147. — Embryo  of  13  mm.  showing  the  Mode  of  Development 
OF  THE  Internal  Mammary  and  Deep  Epigastric  Arteries. — 
(Mall.) 

of  comparative  anatomy  and  on  the  anomalies  which  occur 
in  the  human  body  for  indications  of  what  the  early  devel- 
opment is  likely  to  be.  So  far  as  embryological  observa- 
tions go,  they  confirm  the  conclusions  derived  from  such 
sources. 

Notwithstanding  the  fact  that  the  limbs  are  formed  by 
outgrowths  from  several  segments,  there  is  as  yet  no  evi- 
dence to  show  that  a  corresponding  number  of  segmental 


DEVELOPMENT    OF    ARTERIES    OF    LIMBS.  265 

arteries  take  part  in  the  development  of  their  blood-supply, 
but  it  seems  that  in  both  limbs  the  entire  arterial  system 
is  formed  from  a  single  lateral  branch,  that  of  the  upper 
limb,  the  subclavian,  corresponding  to  the  seventh  cervical 
segment,  while  that  of  the  lower  limb,  the  common  iliac, 
is  probably  the  fifth  lumbar  branch.  In  the  simplest  arrange- 
ment the  subclavian  is  continued  as  a  single  trunk  along  the 
axis  of  the  anterior  limb  as  far  as  the  carpus,  where  it 
divides  into  digital  branches  for  the  fingers.  In  its  course 
through  the  forearm  it  lies  in  the  interval  between  the  radius 
and  ulna,  resting  on  the  interosseous  membrane,  and  in  this 
part  of  its  course  it  may  be  termed  the  arteria  interossea. 
In  the  second  stage  a  new  artery  accompanying  the  median 
nerve  appears,  arising  from  the  main  stem  or  brachial 
artery  a  little  below  the  elbow-joint.  This  may  be  termed 
the  arteria  mediana,  and  as  it  develops  the  arteria  interossea 
gradually  diminishes  in  size,  becoming  finally  the  small 
volar  interosseous  artery  of  the  adult  (Fig.  148),  and  the 
median,  uniting  with  its  lower  end,  takes  from  it  the  digital 
branches  and  becomes  the  principal  stem  of  the  forearm. 

A  third  stage  is  then  ushered  in  by  the  appearance  of  a 
branch  from  the  median  which  forms  the  arteria  nlnaris, 
and  this,  passing  down  the  ulnar  side  of  the  forearm,  unites 
at  the  wrist  with  the  median  to  form  a  superficial  palmar 
arch  from  which  the  digital  branches  arise.  A  fourth  stage 
is  marked  by  the  diminution  of  the  median  artery  until  it 
finally  appears  to  be  a  small  branch  of  the  interosseous,  and 
at  the  same  time  there  develops  from  the  brachial,  at  about 
the  middle  of  the  upper  arm,  what  is  known  as  the  arteria 
radialis  superficialis  (Fig.  148,  rs).  This  extends  down 
the  radial  side  of  the  forearm,  following  the  course  of  the 
radial  nerve,  and  at  the  wrist  passes  upon  the  dorsal  surface 
of  the  hand  to  form  the  dorsal  digital  arteries  of  the  thumb 
and  index  finger.  At  first  this  artery  takes  no  part  in  the 
24 


266 


DEVELOPMENT    OF    ARTERIES    OF    LIMBS, 


formation  of  the  palmar  arches,  but  later  it  gives  rise  to 
the  superficial  volar  branch,  which  usually  unites  with  the 
superficial  arch,  while  from  its  dorsal  portion  a  perforating 
branch  develops  which  passes  between  the  first  and  second 
metacarpal  bones  and  unites  with  a  deep  branch  of  the  ulnar 


Fig.  148. — Diagrams  showing  an  Early  and  a  Late  Stage  in  the 

Development  of  the  Arteries  of  the  Arm. 
b,  Brachial ;  i,  interosseous ;  m^  median  ;  r,  radial ;  rs^  superficial  radial ; 

u,  ulnar. 


to  form  the  deep  arch.  The  fifth  or  adult  stage  is  reached 
by  the  development  from  the  brachial  below  the  elbow  of 
a  branch  (Fig.  148,  r)  which  passes  downward  and  out- 
ward to  unite  with  the  superficial  radial,  whereupon  the 


DEVELOPMENT    OF    ARTERIES    OF    LIMBS.  26/ 

upper  portion  of  that  artery  degenerates  until  it  is  repre- 
sented, only  by  a  branch  to  the  biceps  muscle  (Schwalbe), 
while  the  lower  portion  persists  as  the  adult  radial. 

The  various  anomalies  seen  in  the  arteries  of  the  forearm 
are,  as  a  rule,  due  to  the  more  or  less  complete  persistence  of 
one  or  other  of  the  stages  described  above,  what  is  described, 
for  instance,  as  the  high  branching  of  the  brachial  being  the 
persistence  of  the  superficial  radial. 

In  the  leg  there  is  a  noticeable  difference  in  the  arrange- 
ment of  the  arteries  from  what  occurs  in  the  arm,  in  that 
the  principal  artery  of  the  thigh,  the  femoral,  does  not 
accompany  the  principal  nerve,  the  sciatic.  This  difference 
is  apparently  secondary,  but,  as  in  the  case  of  the  upper 
limb,  it  is  necessary  to  rely  largely  on  the  facts  of  com- 
parative anatomy  and  on  anomalies  which  occur  in  the 
human  body  for  an  idea  of  the  probable  development  of  the 
arteries  of  the  lower  limb.  It  has  already  been  seen  that 
the  common  iliac  artery  is  to  be  regarded  as  a  lateral  branch 
of  the  dorsal  aorta,  and  in  the  simplest  condition  of  the  limb 
arteries  its  continuation,  the  anterior  division  of  the  hypo- 
gastric, passes  down  the  leg  as  a  well-developed  sciatic 
artery  as  far  as  the  ankle  (Fig.  149,  s).  At  the  knee  it 
occupies  the  position  of  the  popliteal  of  adult  anatomy,  and 
below  the  knee  gives  off  a  branch  corresponding  to  the  ante- 
rior tibial  (at)  which,  passing  forward  to  the  extensor  sur- 
face of  the  leg,  quickly  loses  itself  in  the  extensor  muscles. 
The  main  artery  continues  downward  on  the  interosseous 
membrane,  and  some  distance  above  the  ankle  divides  into 
a  strong  anterior  and  a  weaker  posterior  branch ;  the  former 
perforates  the  membrane  and  is  continued  down  the  exten- 
sor surface  of  the  leg  to  form  the  lower  part  of  the  anterior 
tibial  and  the  dorsalis  pedis  arteries,  while  the  latter,  pass- 
ing upon  the  plantar  surface  of  the  foot,  is  lost  in  the  plantar 
muscles.     At  this  stage  the  external  iliac  is  a  secondary 


268 


DEVELOPMENT    OF    ARTERIES    OF    LIMBS. 


branch  of  the  common  ihac,  being  but  poorly  developed  and 


not  extending  as  far  as  the  knee. 


In  the  second  stage  the  external  iliac  artery  increases  in 
size  until  it  equals  the  sciatic,  and  it  now  penetrates  the 
adductor  magnus  muscle  and  unites  with  the  popliteal  por- 
tion of  the  sciatic.     Before  doing  this,  however,  it  gives  off 


/P^ 


dp 


pe 


"A 


^«      at 


■n 


p^ 


Fig.    149. — Diagrams   Illustrating   Stages   in   the  Development   of 

THE  Arteries  of  the  Leg. 
at,   Anterior   tibial ;    dp,   dorsalis    pedis ;    f,    femoral ;   p,   popliteal ;   pe, 

peroneal;    pt,    posterior    tibial;    s,    sciatic    (inferior    gluteal);    sa, 

saphenous. 

a  strong  branch  {so)  which  accompanies  the  long  saphenous 
nerve  down  the  inner  side  of  the  leg,  and,  passing  behind 
the  internal  malleolus,  extends  upon  the  plantar  surface  of 
the  foot,  where  it  gives  rise  to  the  digital  branches.  From 
this  arrangement  the  adult  condition  may  be  derived  by  the 


DEVELOPMENT    OF    THE    VENOUS    SYSTEM.  269 

continued  increase  in  size  of  the  external  iliac  and  its  con- 
tinuation, the  femoral  (/),  accompanied  by  a  reduction  of 
the  upper  portion  of  the  sciatic  and  its  separation  from  its 
popliteal  portion  (/>)  to  form  the  inferior  gluteal  artery  of 
the  adult.  The  continuation  of  the  popliteal  down  the  leg 
is  the  peroneal  artery  (pe)  and  the  upper  perforating  branch 
of  this  unites  with  the  lower  one  to  form  a  continuous  ante- 
rior tibial,  the  lower  connection  of  which  with  the  peroneal 
persists  in  part  as  the  anterior  peroneal  artery.  A  new 
branch  arises  from  the  upper  part  of  the  peroneal  and  passes 
down  the  back  of  the  leg  to  unite  with  the  lower  part  of 
the  arteria  saphena,  forming  the  posterior  tibial  artery  (pt), 
and  the  upper  part  of  the  saphenous  becomes  much  reduced, 
persisting  as  the  superficial  branch  of  the  art.  genu  suprema 
and  a  rudimentary  chain  of  anastomoses  which  accompany 
the  long  saphenous  nerve. 

The  Development  of  the  Venous  System. — The  earli- 
est veins  to  develop  are  those  which  accompany  the  first- 
formed  arteries,  the  omphalo-mesenterics  and  umbilicals, 
but  it  will  be  more  convenient  to  consider  first  the  veins 
which  carry  the  blood  from  the  body  of  the  embryo  back 
to  the  heart.  These  make  their  appearance,  while  the 
heart  is  still  in  the  pharyngeal  region,  as  two  pairs  of  longi- 
tudinal trunks,  the  anterior  and  posterior  cardinal  veins, 
into  which  lateral  branches,  arranged  more  or  less  seg- 
mentally,  open.  The  anterior  cardinals  appear  somewhat 
earlier  than  the  posterior  and  form  the  internal  jugular 
veins  of  adult  anatomy.  Each  vein  extends  forward  from 
the  heart  at  the  side  of  the  notochord  and  is  continued  on 
the  under  surface  of  the  brain,  lying  medial  to  the  roots  of 
the  cranial  nerves.  Later  sprouts  arising  from  the  vein 
form  loops  around  the  nerve  roots  and  the  portion  of  the 
loops  formed  by  the  original  vein  then  disappear,  so  that 
the  vessel  now  lies  lateral  to  nerve  roots,  except  in  the  case 


270 


DEVELOPMENT    OF    THE    VENOUS    SYSTEM. 


of  the  trigeminus,  where  the  original  vessel  persists  to  form 
the  cavernous  sinus.  From  the  vena  capitis  lateralis  so 
formed  three  veins,  an  anterior,  a  middle  and  a  posterior 
cerebral,  pass  to  the  brain,  the  anterior  cerebral  together 
with  the  ophthalmic  vein  opening  into  the  anterior  end  of 
the  cavernous  sinus,  the  middle  cerebral  into  the  posterior 
extremity  of  the  same  sinus  and  the  posterior  cerebral  into 


Fig.  150. — Reconstruction  of  the  Head  of  a  Human  Embryo  of  9  mm. 
SHOWING  the  Cekebral  Veins. 

acv,  anterior  cerebral  vein;  au,  auditory  vesicle;  cs,  cavernous  sinus; 
Fa,  facial  nerve;  mcv.  middle  cerebral  vein;  pcv,  posterior  cere- 
bral vein;  U;  trigeminal  nerve;  vcl^  lateral  cerebral  vein. —  (Mall.) 


the  vena  capitis  lateralis  behind  the  ear  vesicle  (Fig.  150). 
The  branches  of  the  anterior  cerebral  vein  extending  over 
the  cerebral  hemispheres  unite  with  their  fellows  of  the 
opposite  side  to  form  a  longitudinal  trunk,  the  superior 
sagittal  sinus,  lying  between  the  two  cerebral  hemisphere: 


DEVELOPMENT    OF    THE    VENOUS    SYSTEM. 


271 


At  first  this  sinus  drains  by  way  of  the  anterior  cerebral 
vein  (Fig.  151,  A),  but  as  the  cerebral  hemispheres  increase 
in  size  it  is  gradually  carried  backward  and  makes  connec- 
tions first  with  the  middle  cerebral  and  later  with  the  pos- 
terior cerebral  vein  (Fig.  151,  B  and  C) ,  each  of  these 
becoming  in  turn  the  principal  drainage  of  the  sinus.     The 


Fig.  151. — Diagrams  showing  the  Arrangement  of  the  Cerebral 
Veins  in  Embryos  of  (A)  the  Fifth  Week,  (B)  the  Begin- 
ing  of  the  Third  Month  and  in  (C)  an  Older- Fetus. 

acVj  anterior  cerebral  vein ;  cs,  cavernous  sinus ;  ils,  inferior  sagittal 
sinus ;  Inf.  Pet.,  inferior  petrosal  sinus ;  Is,  transverse  sinus ;  ov, 
ophthalmic  vein ;  sis,  superior  sagittal  sinus;  sps,  spheno-parietal 
sinus;  sr,  straight  sinus;  ss,  middle  cerebral  vein  (Sylvian);  sup. 
pet,  superior  petrosal  sinus ;  th,  torcular  Herophili ;  v,  trigeminal 
nerve ;  vca,  anterior  cerebral  vein ;  vcl,  lateral  cerebral  vein ;  vcm, 
middle  cerebral  vein;  vcp,  posterior  cerebral  vein;  vg,  vein  of 
Galen;  vj,  internal  jugular. —  (Mall.) 


connections  which  join  the  veins  to  the  sinus  become  the 
proximal  portion  of  the  transverse  sinus,  the  posterior  cere- 


2/2  DEVELOPMENT    OF    THE    VENOUS    SYSTEM. 

bral  vein  itself  becoming  the  distal  portion,  the  middle 
cerebral  vein  becomes  the  superior  petrosal  sinus,  while  the 
anterior  cerebral  vein  persists  as  the  middle  cerebral  vein 
of  adult  anatomy  (Fig.  151,  C).  Additional  sprouts  from 
the  terminal  portion  of  the  superior  sagittal  sinus  give  rise 
to  the  straight  and  inferior  sagittal  sinuses,  and,  after  the 
disappearance  of  the  vena  capitis  lateralis,  a  new  stem  devel- 
ops between  the  cavernous  and  transverse  sinuses,  passing 
medial  to  the  ear  vesicle,  and  forms  the  inferior  petrosal 
sinus  (Fig.  151,  C).  This  joins  the  transverse  sinus  at 
the  jugular  foramen  and  from  this  junction  onwards  the 
anterior  cardinal  vein  may  now  be  termed  the  internal  jugu- 
lar vein. 

Passing  backward  from  the  jugular  foramen  the  internal 
jugular  veins  unite  with  the  posterior  cardinals  to  form  on 
each  side  a  common  trunk,  the  ductus  Cuvieri,  and  then 
passing  transversely  toward  the  median  line  open  into  the 
sides  of  the  sinus  venosus.  So  long  as  the  heart  retains  its 
original  position  in  the  pharyngeal  region  the  jugular  is  a 
short  trunk  receiving  lateral  veins  only  from  the  upper- 
most segments  of  the  neck  and  from  the  occipital  segments, 
the  remaining  segmental  veins  opening  into  the  inferior 
cardinals.  As  the  heart  recedes,  however,  the  jugulars 
become  more  and  more  elongated  and  the  cervical  lateral 
veins  shift  their  communication  from  the  cardinals  to  the 
jugulars,  until,  when  the  subclavians  have  thus  shifted,  the 
jugulars  become  much  larger  than  the  cardinals.  When 
the  sinus  venosus  is  absorbed  into  the  wall  of  the  right 
auricle,  the  course  of  the  left  Cuvierian  duct  becomes  a 
little  longer  than  that  of  the  right,  and  from  the  left  jugu- 
lar, at  the  point  where  it  is  joined  by  the  left  subclavian, 
a  branch  arises  which  extends  obliquely  across  to  join  the 
right  jugular,  forming  the  left  innominate  vein.  When 
this  is  established,  the  connection  between  the  left  jugular 


DEVELOPMENT    OF    THE    VENOUS    SYSTEM. 


273 


and  Cuvierian  duct  is  dissolved,  the  blood  from  the  left  side 
of  the  head  and  neck  and  from  the  left  subclavian  vein  pass- 
ing over  to  empty  into  the  right  jugular,  whose  lower  end, 
together  with  the  right  Cuvierian  duct,  thus  becomes  the 
superior  vena  cava.  The  left  Cuvierian  duct  persists,  form- 
ing with  the  left  horn  of  the  sinus  venosus  the  coronary- 
sinus. 


Fig.    152. — Diagrams    showing   the   Development   of   the   Superior 

Vena  Cava. 

a,  Azygos  vein;  cs,  coronary  sinus;  ej,  external  jugular;  h,  hepatic 
vein;  ij,  internal  jugular;  inr  and  inl,  right  and  left  innominate 
veins ;  s,  subclavian ;  vci  and  vcs,  inferior  and  superior  venae  cavse. 

The  external  jugular  vein  develops  somewhat  later  than 
the  internal.  The  facial  vein,  which  primarily  forms  the 
principal  affluent  of  this  stem,  passes  at  first  into  the  skull 
along  with  the  fifth  nerve  and  communicates  with  the  in- 
ternal jugular  system,  but  later  this  original  communica- 
tion is  broken  and  the  facial  vein,  uniting  with  other  super- 
ficial veins,  passes  over  the  jaw  and  extends  down  the  neck 
as  the  external  jugular.  Later  still  the  facial  anastomoses 
with  the  ophthalmic  at  the  inner  angle  of  the  eye  and  also 
makes  connections  with  the  internal  jugular  just  after  it 
has  crossed  the  jaw,  and  so  the  adult  condition  is  acquired. 


2/4  DEVELOPMENT    OF    THE    VENOUS    SYSTEM. 

It  is  interesting  to  note  that  in  many  of  the  lower  mammals 
the  external  jugular  becomes  of  much  greater  importance  than 
the  internal,  the  latter  in  some  forms,  indeed,  eventually  disap- 
pearing and  the  blood  from  the  interior  of  the  skull  emptying 
by  means  of  anastomoses  which  have  developed  into  the  exter- 
nal jugular  system.  In  man  the  primitive  condition  is  re- 
tained, but  indications  of  a  transference  of  the  intracranial 
blood  to  the  external  jugular  are  seen  in  the  emissary  veins. 

The  posterior  cardinal  veins,  or,  as  they  may  more  simply 
be  termed,  the  cardinals,  extend  backward  from  their  union 
with  the  jugulars  along  the  sides  of  the  vertebral  column, 
receiving  veins  from  the  mesentery  and  also  from  the  vari- 
ous lateral  segmental  veins  of  the  neck  and  trunk  regions, 
with  the  exception  of  that  of  the  first  cervical  segment 
which  opens  into  the  jugular.  Later,  however,  as  already 
described  (p.  273),  the  cervical  veins  shift  to  the  jugulars, 
as  do  also  the  first  and  second  thoracic  (intercostal)  veins, 
but  the  remaining  intercostals,  together  with  the  lumbars 
and  sacrals,  continue  to  open  into  the  cardinals.  In  addi- 
tion, the  cardinals  receive  in  early  stages  the  veins  from  the 
primitive  kidneys  (mesonephros),  which  are  exceptionally 
large  in  the  human  embryo,  but  when  they  are  replaced 
later  on  by  the  permanent  kidneys  (metanephros)  their 
afferent  veins  undergo  a  reduction  in  number  and  size,  and 
this,  together  with  the  shifting  of  the  upper  lateral  veins, 
produces  a  marked  diminution  in  the  size  of  the  cardinals. 
The  changes  by  which  they  acquire  their  final  arrangement 
are,  however,  so  intimately  associated  with  the  development 
of  the  inferior  vena  cava  that  their  description  may  be  con- 
veniently postponed  until  the  history  of  the  omphalo-mesen- 
teric  and  umbilical  veins  has  been  presented. 

The  omphalo-inesenteric  veins  are  two  in  number,  a  right 
and  a  left,  and  pass  in  along  the  yolk-stalk  until  they  reach 
the  embryonic  intestine,  along  the  sides  of  which  they  pass 
forward  to  unite  with  the  corresponding  umbilical  veins. 


DEVELOPMENT    OF    THE    VENOUS    SYSTEM. 


275 


These  are  represented  in  the  belly-stalk  by  a  single  venous 
trunk  which,  when  it  reaches  the  body  of  the  embryo, 
divides  into  two  stems  which  pass  forward,  one  on  each 
side  of  the  umbilicus,  and  thence  on  each  side  of  the  median 
line  of  the  ventral  abdominal  wall,  to  form  with  the  corre- 
sponding omphalo-mesenteric  veins  common  trunks  which 
open  into  the  ductus  Cuvieri.  As  the  liver  develops  it 
comes  into  intimate  relation  with  the  omphalo-mesenteric 
veins,  which  receive  numerous  branches  from  its  substance 
and,  indeed,  seem  to  break  up  into  a  network  (Fig.  153,  A) 


DC, 


D.C. 


D.Cr 


^I>C 


z>.rA 


VoTn.s.  Vo.TTtd.      ^Vo.Tns 


Fig.    153.— Diagrams    Illustrating    the    Transformations    of    the 

Omphalomesenteric  and  Umbilical  Veins. 
D.C,  Ductus   Cuvieri;  D.V.A,   ductus  venosus;    V.o.m.d   and    V.o.m.s, 

right   and   left  omphalo-mesenteric   veins;    V.ii.d   and    V.u.s,   right 

\nd  left  umbilical  veins. —  (Hochstetter.) 

traversing  the  liver  substance  and  uniting  again  to  form 
two  stems  which  represent  the  original  continuations  of  the 
omphalo-mesenterics.  From  the  point  where  the  common 
trunk  formed  by  the  right  omphalo-mesenteric  and  umbili- 
cal veins  opens  into  the  Cuvierian  duct  a  new  vein  develops, 
passing  downward  and  to  the  left  to  unite  with  the  left 


276 


DEVELOPMENT    OF    THE    VENOUS    SYSTEM. 


omphalo-mesenteric ;  this  is  the  ductus  venosus  (Fig.  153, 
B,  DVA).  In  the  meantime  three  cross-connections  have 
developed  between  the  two  omphalo-mesenteric  veins,  two 
of  which  pass  ventral  and  the  other  dorsal  to  the  intestine, 
so  that  the  latter  is  surrounded  by  two  venous  loops  (Fig. 
154,  A),  and  a  connection  is  developed  between  each  um- 
bilical vein  and  the  corresponding  omphalo-mesenteric  (Fig. 
153,  B),  that  of  the  left  side  being  the  larger  and  uniting 
with  the  omphalo-mesenteric  just  where  it  is  joined  by  the 


Fig.  154. — A,  The  Venous  Trunks  of  an  Embryo  of  5  mm.  seen  from 
THE  Ventral  Surface;  B,  Diagram  Illustrating  the  Trans- 
formation TO  the  Adult  Condition. 

Vcd  and  Vcs,  right  and  left  superior  venas  cavse;  Vj,  jugular  vein; 
V.om,  omphalo-mesenteric  vein;  Vp,  vena  porta;  Vu,  umbilical 
vein  (lower  part)  ;  Vn,  umbilical  vein  (upper  part)  ;  Vud  and  Vus, 
right   and    left   umbilical    veins    (lower   parts). —  (His.) 


ductus  venosus  so  as  to  seem  to  be  the  continuation  of  this 
vessel  (Fig.  153,  C).  When  these  connections  are  com- 
plete, the  upper  portions  of  the  umbilical  veins  degenerate 
(Fig.  154),  and  now  the  right  side  of  the  lower  of  the  two 
omphalo-mesenteric  loops  which  surround  the  intestine  dis- 


DEVELOPMENT    OF    THE    VENOUS    SYSTEM.  2/7 

appears,  as  does  also  that  portion  of  the  left  side  of  the 
upper  loop  which  intervenes  between  the  middle  cross-con- 
nection and  the  ductus  venosus,  and  so  there  is  formed  from 
the  omphalo-mesenteric  veins  the  vena  porta. 

While  these  changes  have  been  progressing  the  right 
umbilical  vein,  originally  the  larger  of  the  two  (Fig.  153, 
A  and  B,  V.ii.d),  has  become  very  much  reduced  in  size 
and,  losing  its  connection  with  the  left  vein  at  the  umbilicus, 
forms  a  vein  of  the  ventral  abdominal  wall  in  which  the 
blood  now  flows  from  above  downward.  The  left  umbili- 
cal now  forms  the  only  route  for  the  return  of  blood  from 
the  placenta,  and  appears  to  be  the  direct  continuation  of 
the  ductus  venosus  (Fig.  154,  C),  into  which  open  the 
hepatic  veins,  returning  the  blood  distributed  by  the  portal 
vein  to  the  substance  of  the  liver. 

Returning  now  to  the  posterior  cardinal  veins,  it  has 
been  found  that  in  the  rabbit  the  branches  which  come  to 
them  from  the  mesentery  anastomose  longitudinally  to  form 
a  vessel  lying  parallel  and  slightly  ventral  to  each  cardinal. 
These  may  be  termed  the  suh cardinal  veins  (Lewis),  and 
in  their  earliest  condition  they  open  at  either  end  into  the 
corresponding  cardinal,  with  which  they  are  also  united  by 
numerous  cross-branches.  Later,  in  rabbits  of  8.8  mm., 
these  cross-branches  begin  to  disappear  and  give  place  to 
a  large  cross-branch  situated  immediately  below  the  origin 
of  the  superior  mesenteric  artery,  and  at  the  same  point  a 
cross-branch  between  the  two  subcardinals  also  develops. 
The  portion  of  the  right  subcardinal  which  is  anterior  to 
the  cross-connection  now  rapidly  enlarges  and  unites  with 
the  ductus  venosus  about  where  the  hepatic  veins  open  into 
that  vessel  (Fig.  155,  A),  and  the  portion  of  each  posterior 
cardinal  immediately  above  the  entrance  of  the  renal  veins 
degenerates,  so  that  all  the  blood  received  by  the  posterior 
portions  of  the  cardinals  is  returned  to  the  heart  by  way  of 


2/8 


DEVELOPMENT    OF    THE    VENOUS    SYSTEM. 


the  right  subcardinal,  its  cross-connections,  and  the  upper 
part  of  the  ductus  venosus. 

When  this  is  accompHshed  the  lower  portions  of  the  sub- 
cardinals  disappear,  while  the  portions  above  the  large  cross- 
connection  persist,  greatly  diminished  in  size,  as  the  supra- 
renal veins  (Fig.  155,  B). 

In  the  early  stages  the  veins  which  drain  the  posterior 


Fig.  155. — Diagrams  Illustrating  the  Development  of  the  Inferior 

Vena  Cava. 

The  cardinal  veins  and  ductus'  venosus  are  black,  the  subcardinal  sys- 
tem blue,  and  the  supracardinal  yellow.  cs,  coronary  sinus;  dv, 
ductus  venosus;  il,  iliac  vein;  r,  renal;  s,  internal  spermatic j  scl, 
subclavian ;  sr,  suprarenal ;  va,  azygos ;  vha,  hemiazygos ;  vi,  in- 
nominate;  vj,  internal  jugular. 

abdominal  walls  empty  into  the  posterior  cardinals,  and 
later  they  form,  in  the  region  of  the  kidney  on  each  side,  a 
longitudinal  anastomosis  which  opens  at  either  extremity 
into  the  posterior  cardinal.     The  ureter  thus  Ijecomes  sur- 


DEVELOPMENT    OF    THE    VENOUS    SYSTEM.  2/9 

rounded  by  a  venous  ring,  the  dorsal  limb  of  which  is 
formed  by  the  new  longitudinal  anastomosis,  which  has 
been  termed  the  supracardinal  vein  (McClure  and  Hunt- 
ington), while  the  ventral  limb  is  formed  by  a  portion  of 
the  posterior  cardinal  (Fig.  155,  B).  Still  later  the  ven- 
tral limb  of  the  loop  disappears  and  the  dorsal  supracardinal 
limb  replaces  a  portion  of  the  more  primitive  posterior  car- 
dinal. An  anastomosis  now  develops  between  the  right 
and  left  cardinals  at  the  point  where  the  iliac  veins  open  into 
them  (Fig.  155,  B),  and  the  portion  of  the  left  cardinal 
which  intervenes  between  this  anastomosis  and  the  entrance 
of  the  internal  spermatic  vein  disappears,  the  remainder  of 
it,  as  far  forward  as  the  renal  vein,  persisting  as  the  upper 
part  of  the  left  internal  spermatic  vein,  which  thus  comes 
to  open  into  the  renal  vein  instead  of  into  the  vena  cava 
as  does  the  corresponding  vein  of  the  right  side  of  the  body 
(Fig.  155,  C,  s).  The  renal  veins  originally  open  into  the 
cardinals  at  the  point  where  these  are  joined  by  the  large 
cross-connection,  and  when  the  lower  part  of  the  left  car- 
dinal disappears,  this  cross-connection  forms  the  proximal 
part  of  the  left  renal  vein,  which  consequently  receives  the 
left  suprarenal  (Fig.  155,  C). 

The  observations  upon  which  the  above  description  is 
based  have  been  made  upon  the  rabbit,  but  it  seems  prob- 
able from  the  partial  observations  that  have  been  made  that 
similar  changes  occur  also  in  the  human  embryo.  It  will 
be  noted  from  what  has  been  said  that  the  inferior  vena 
cava  is  a  composite  vessel,  consisting  of  at  least  four  ele- 
ments:  (i)  the  proximal  part  of  the  ductus  venosus;  (2) 
the  anterior  part  of  the  right  subcardinal;  (3)  the  right 
supracardinal;  and  (4)  the  posterior  part  of  the  right 
cardinal. 

Recent  observations  by  McClure  and  Huntington  on  the 
development  of  the  veins  of  the  cat  show,  however,  that 


28o 


DEVELOPMENT    OF    THE    VENOUS    SYSTEM. 


in  this  form  the  process  is  somewhat  different  from  what 
obtains  in  the  rabbit.  The  differences  depend  principally 
upon  the  greater  development  of  the  supracardinal  veins, 
these  vessels  extending  far  anterior  to  the  subcardinal  cross- 
connection,  and  forming  the  dorsal  limb  of  a  very  wide 
ureteric  venous  ring  (Fig.  156,  A).  During  development 
the  two  supracardinal  limbs  of  the  ureteric  ring  approach 


Fig.  156. — Diagrams  Illustrating  the  Development  of  the  Inferior 
Vena  Cava  in  the  Cat. 

The  cardinal  veins  and  ductus  venosus  are  black,  the  subcardinal  sys- 
tem blue,  and  the  supracardinal  yellow.  cs,  coronary  sinus ;  dv, 
ductus  venosus ;  il,  iliac  vein ;  r,  renal ;  s,  internal  spermatic ;  scl, 
subclavian;  sr,  suparenal ;  va,  azygos;  vi,  innominate;  vj,  internal 
jugular    (adapted    from    McClure). 

each  other,  and  finally  unite  in  the  median  line,  whereupon 
both  right  and  left  post-renal  portions  of  the  cardinals  dis- 
appear and  the  post-renal  portion  of  the  vena  cava  is  formed 
by  the  fused  supracardinals  (Fig.  156,  B  and  C). 

The  complicated  development  of  the  inferior  vena  cava  nat- 
urally gives   rise  to  numerous  anomalies  of  the  vein  due  to 


DEVELOPMENT    OF    THE   VENOUS    SYSTEM.  28 1 

inhibitions  of  its  development.  These  anomahes  affect  espe- 
cially the  post-renal  portion,  a  persistence  of  both  cardinals 
(interpreting  the  conditions  in  the  terms  of  what  occurs  in  the 
rabbit)  giving  rise  to  a  double  post-renal  cava,  or  a  persistence 
of  the  left  cardinal  and  the  disappearance  of  the  right  to  a 
vena  cava  situated  on  the  left  side  of  the  vertebral  column  and 
crossing  to  the  right  by  way  of  the  left  renal  vein.  So,  too, 
the  occurrence  of  accessory  renal  veins  passing  dorsal  to  the 
ureter  is  explicable  in  the  supposition  that  they  represent  por- 
tions of  the  supracardinal  system  of  veins. 

It  has  already  been  noted  that  the  portions  of  the  pos- 
terior cardinals  immediately  anterior  to  the  entrance  of  the 
renal  veins  disappear.  The  upper  part  of  the  right  vein 
persists,  how^ever,  and  becomes  the  vena  asygos  of  the  adult, 
while  the  upper  portion  of  the  left  vein  sends  a  cross-branch 
over  to  unite  with  the  azygos  and  then  separates  from  the 
coronary  sinus  to  form  the  vena  hemiazygos.  At  least  this 
is  what  is  described  as  occurring  in  the  rabbit.  In  the  cat, 
however,  only  the  very  uppermost  portion  of  the  right  pos- 
terior cardinal  persists  and  the  greater  portion  of  the  azygos 
and  perhaps  the  entire  hemiazygos  vein  is  formed  from  the 
prerenal  portions  of  the  supracardinal  veins,  the  right  one 
joining  oh  to  the  small  persisting  upper  portion  of  the  right 
posterior  cardinal,  while  the  cross-connection  between  the 
hemiazygos  and  azygos  represents  one  of  the  originally 
numerous  cross-connections  between  the  supracardinals. 

The  ascending  lumbar  veins,  frequently  described  as  the 
commencements  of  the  azygos  veins,  are  in  reality  secondary 
formations  developed  by  the  anastomoses  of  anteriorly  and 
posteriorly  directed  branches  of  the  lumbar  veins. 

The  Development  of  the  Veins  of  the  Limbs. — The  devel- 
opment of  the  limb  veins  of  the  human  embryo  requires 
further  investigation,  but  from  a  comparison  of  what  is 
known  with  what  has  been  observed  in  rabbit  embryos  it 
may  be  presumed  that  the  changes  which  take  place  are 
somewhat  as  follows :  In  the  anterior  extremity  the  blood 
25 


282  DEVELOPMENT    OF    THE    VENOUS    SYSTEM. 

brought  to  the  limb  is  collected  by  a  vein  which  passes  dis- 
tally  along  the  radial  border  of  the  limb  bud,  around  its 
distal  border,  and  proximally  along  its  ulnar  border  to  open 
into  the  anterior  cardinal  vein;  this  is  the  primary  ulnar 
vein.  Later  a  second  vein  grows  out  from  the  external 
jugular  along  the  radial  border  of  the  limb,  representing  the 
cepJialic  vein  of  the  adult,  and  on  its  appearance  the  digital 
veins,  which  were  formed  from  the  primary  ulnar  vein, 
become  connected  with  it,  and  the  distal  portion  of  the  pri- 
mary ulnar  vein  disappears.  Its  proximal  portion  persists, 
however,  to  form  the  basilic  vein,  from  which  the  brachial 
vein  and  its  continuation,  the  ulnar  vein,  are  developed, 
while  the  radial  vein  develops  as  an  outgrowth  from  the 
cephalic,  which  at  an  early  stage  secures  an  opening  into 
the  axillary  vein,  its  original  communication  with  the  exter- 
nal jugular  forming  the  jugulo-cephalic  vein. 

In  the  lower  limb  a  primary  fibular  vein,  exactly  compar- 
able to  the  primary  ulnar  of  the  arm,  surrounds  the  distal 
border  of  the  limb-bud  and  passes  up  its  fibular  border  to 
open  with  the  posterior  cardinal  vein.  The  further  devel- 
opment in  the  lower  limb  differs  considerably,  however, 
from  that  of  the  upper  limb.  From  the  primary  fibular  vein 
an  anterior  tibial  vein  grows  out,  which  receives  the  digital 
branches  from  the  toes,  and  from  the  posterior  cardinal, 
anterior  to  the  point  where  the  primary  fibular  opens  into  it, 
a  vein  grows  down  the  tibial  side  of  the  leg,  forming  the 
long  saphenous  vein.  From  this  the  femoral  vein  is  formed 
and  from  it  the  posterior  tibial  vein  is  continued  down  the 
leg.  An  anastomosis  is  formed  between  the  femoral  and 
the  primary  fibular  veins  at  the  level  of  the  knee  and  the 
proximal  portion  of  the  latter  vein  then  becomes  greatly 
reduced,  while  its  distal  portion  possibly  persists  as  the  small 
saphenous  vein  (Hochstetter). 

The  Pulmonary    Veins. — The  development  of   the  pul- 


THE    FETAL    CIRCULATION. 


283 


monary  veins  has  already  been  described  in  connection  with 
the  development  of  the  heart  (see  p.  247). 

The  Fetal  Circulation. — During  fetal  life  while  the  pla- 
centa is  the  sole  orran  in  which  occur  the  changres  in  the 


Fig.   157. — The  Fetal  Circulation. 
ao,  Aorta;   a.pu,   pulmonary  artery;   au,   umbilical   artery;   da,   ductus 
arteriosus ;  dv,  ductus  venosus ;  int,  intestine ;  vci  and  vcs,  inferior 
and  superior  vena  cava ;  vJi,  hepatic  vein ;  vp,  vena  portse ;  v.pu, 
pulmonary  vein;  vu,  umbilical  vein. —  {From  Kollmann.) 

blood  on  which  the  nutrition  of  the  embryo  depends,  the 
course  of  the  blood  is  necessarily  somewhat  different  from 
what  obtains  in  the  child  after  birth.     Taking  the  placenta 


284  THE    FETAL    CIRCULATION. 

as  the  starting-point,  the  blood  passes  along  the  umbilical 
vein  to  enter  the  body  of  the  fetus  at  the  umbilicus,  whence 
it  passes  forward  in  the  free  edge  of  the  anterior  mesentery 
(see  p.  341)  until  it  reaches  the  liver.  Here,  owing  to  the 
anastomoses  between  the  umbilical  and  omphalo-mesenteric 
veins,  a  portion  of  the  blood  traverses  the  substance  of  the 
liver  to  open  by  the  hepatic  veins  into  the  inferior  vena 
cava,  while  the  remainder  passes  on  through  the  ductus 
venosus  to  the  cava,  the  united  streams  opening  into  the 
right  atrium.  This  blood,  whose  purity  is  only  slightly 
reduced  by  mixture  with  the  blood  returning  from  the  infe- 
rior vena  cava,  is  prevented  from  passing  into  the  right  ven- 
tricle by  the  Eustachian  valve,  which  directs  it  to  the  fora- 
men ovale,  and  through  this  it  passes  into  the  left  atrium, 
thence  to  the  left  ventricle,  and  so  out  by  the  systemic  aorta. 

The  blood  which  has  been  sent  to  the  head,  neck,  and 
upper  extremities  is  returned  by  the  superior  vena  cava  also 
into  the  right  atrium,  but  this  descending  stream  opens  into 
the  atrium  to  the  right  of  the  annulus  of  Vieussens  (see 
Fig.  135)  and  passes  directly  to  the  right  ventricle  without 
mingling  to  any  great  extent  with  the  blood  returning  by 
way  of  the  inferior  cava.  From  the  right  ventricle  this 
blood  passes  out  by  the  pulmonary  artery;  but  the  lungs  at 
this  period  are  collapsed  and  in  no  condition  to  receive  any 
great  amount  of  blood,  and  so  the  stream  passes  by  way 
of  the  ductus  arteriosus  into  the  systemic  aorta,  meeting 
there  the  placental  blood  just  below  the  point  where  the  left 
subclavian  artery  is  given  off.  From  this  point  onward  the 
aorta  contains  only  mixed  blood,  and  this  is  distributed  to 
the  walls  of  the  thorax  and  abdomen  and  to  the  lungs  and 
abdominal  viscera,  the  greater  part  of  it,  however,  passing 
off  in  the  hypogastric  arteries  and  so  out  again  to  the 
placenta. 

It  will  be  perceived  that  although  no  portion  of  the  body 


DEVELOPMENT    OF    THE   LYMPHATIC    SYSTEM.  285 

receives  absolutely  pure  placental  blood,  yet  the  quality  of 
that  which  is  supplied  to  the  liver,  heart,  head,  neck,  and 
upper  limbs  is  much  better  than  that  distributed  by  the 
branches  arising  from  "the  aorta  below  the  union  of  the 
ductus  arteriosus.  Hence  it  is  that  the  anterior  portions  of 
the  fetus  are  much  better  developed  than  the  posterior. 

At  birth  the  lungs  at  once  assume  their  functions,  and 
on  the  cutting  of  the  umbilical  cord  all  communication  with 
the  placenta  ceases.  Shortly  after  birth  the  foramen  ovale 
closes  more  or  less  perfectly,  and  the  ductus  arteriosus  di- 
minishes in  size  as  the  pulmonary  arteries  increase,  and 
becomes  eventually  converted  into  a  fibrous  cord.  The 
hypogastric  arteries  diminish  greatly,  and  after  they  have 
passed  the  bladder  are  also  reduced  to  fibrous  cords,  a  fate 
likewise  shared  by  the  umbilical  vein,  which  becomes  con- 
verted into  the  round  ligament  of  the  liver. 

The  Development  of  the  Lymphatic  System. — It  has 
already  been  seen  (p.  237)  that  the  lymphocytes  first  make 
their  appearance  in  the  tissues  surrounding  the  early  blood- 
vessels, but  opinions  differ  as  to  their  exact  origin.  Accord- 
ing to  some  observers,  they  are  formed  by  modification  of 
mesenchyme  cells,  while  others  believe  that  they  have  evi- 
dence that  the  lymphocytes  of  the  intestinal  and  tonsillar 
lymph-nodes  are  derived  from  the  intestinal  and  tonsillar 
epithelium,  and  quite  recently  it  has  been  maintained  that 
the  epithelial  cells  which  form  the  thymus  body  in  fishes  are 
directly  transformed  into  lymphocytes.  Which  view  will 
prove  correct  must  be  left  for  future  observations  to  decide. 

The  observations  upon  the  origin  of  the  lymphatic  vessels 
are  at  present  somewhat  discordant.  According  to  the 
observations  of  Sabin,  the  vessels  are  first  distinguishable 
in  pig  embryos  of  14.5  cm.  as  two  small  sacs  or  lymph 
hearts,  which  arise,  one  on  each  side,  as  outgrowths  from 
near  the  junction  of  the  subclavian  and  jugular  veins,  the 


286  DEVELOPMENT    OF    THE    LYMPHATIC    SYSTEM. 

opening  of  the  sac  into  the  veins  being  guarded  by  a  valve 
due  to  the  obHque  direction  taken  by  the  outgrowth.  From 
each  lymph  heart  branches,  which  anastomose  and  radiate 
in  all  directions,  grow  outward  toward  the  skin,  which  they 
reach  in  embryos  of  about  i8  mm.,  and  in  later  stages  con- 
tinue to  extend  in  a  radiating  manner  until  they  form  a  sub- 
cutaneous network  over  the  anterior  half  of  the  body.  In 
the  meantime  the  lymph  hearts  have  separated  from  their 
points  of  origin  (Fig.  158,  A,  ALH),  with  which,  however, 
they  remain  connected  by  a  duct,  and  from  this  a  branch 
grows  backward,  following  the  line  of  the  vagus  nerve  (Fig. 
158,  A,  TD).  The  branch  on  the  left  side  soon  meets  with 
the  aorta  and,  using  this  as  a  guide,  grows  more  rapidly 
than  its  fellow  on  the  right  and  becomes  the  thoracic  duct, 
or,  rather,  since  it  divides  just  before  it  reaches  the  aorta 
and  sends  a  branch  backward  on  either  side  of  that  vessel, 
it  gives  rise  to  tzvo  thoracic  ducts  (Fig.  158,  B). 

In  embryos  of  20  mm.  a  second  pair  of  lymph  hearts  de- 
velops at  the  junction  of  the  sciatic  veins  with  the  cardinals 
(Fig.  158,  A,  PLH),  and  from  these  branches  grow  toward 
the  surface  and  radiate  subcutaneously,  similarly  to  those 
from  the  anterior  hearts,  with  which  they  eventually  unite. 
The  thoracic  ducts,  continuing  to  elongate  backward,  dilate 
opposite  the  kidneys  to  form  two  receptacida  chyli  (Fig. 
158,  B,  RC)  and  still  more  posteriorly  unite  with  the  pos- 
terior lymph  hearts,  which  then  separate  completely  from 
the  veins  from  which  they  originated. 

In  later  stages  branches,  arising  as  outgrowths  from  the 
thoracic  ducts,  gradually  invade  the  mesentery  and  the 
various  organs,  following  in  general  the  course  of  the 
arteries,  as  do  also  the  branches  which  pass  to  the  limbs  to 
form  their  deep  lymphatics ;  the  superficial  branches,  on  the 
contrary,  follow  essentially  the  course  of  the  veins.  The 
lymph  hearts  as  development  proceeds  become  transformed 


PEVELOPMENT   OF   THE   LYMPHATIC    SYSTEM. 


287 


into  lymph  nodes,  and  at  various  points  in  the  system  minute 
plexuses  arise.  Up  to  this  stage  no  valves  are  present  in 
the  vessels,  and  the  development  of  these  has  yet  to  be 


Fig.  158. — DiAGiiAMS  showing  the  Arrangement  of  the  Lymphatic 
Vessels  in  Pig  Embryos  of  (A)  20  mm.  and  (B)  40  mm. 

ACV,  Jugular  vein;  ADR,  suprarenal  body;  ALH,  anterior  lymph 
heart;  Ao,  aorta;  Arm  D,  deep  lymphatics  to  the  arm;  D,  dia- 
phragm; Du,  branches  to  duodenum;  FV,  femoral  vein;  H, 
branches  to  heart;  K,  kidney;  LegD,  deep  lymphatics  to  leg;  Lu, 
branches  to  lung;  MP,  branches  to  mesenteric  plexus;  CE,  branch 
to  oesophagus;  PCV,  cardinal  vein;  PLH,  posterior  lymph  heart; 
RC,  receptaculum  chyli ;  RLD,  right  lymphatic  duct;  ScV,  sub- 
clavian vein ;  SV,  sciatic  vein ;  St,  branches  to  stomach ;  TD, 
thoracic  duct;  WB,  Wolffian  body. —  (Sabin.) 


288  DEVELOPMENT    OF    THE   LYMPHATIC    SYSTEM. 

studied,  as  has  also  the  final  transformation  of  the  condition 
described  into  that  found  in  the  adult. 

Lewis,  from  his  observations  on  rabbit  embryos,  concludes 
that  the  lymphatics  arise  as  outgrowths  from  the  veins,  but 
the  outgrowths  occur  not  only  at  the  points  indicated  by 
Sabin,  but  also  at  other  points,  as,  for  instance,  near  the  root 
of  the  external  mammary  vein  and  in  the  vicinity  of  the 
azygos,  gastric  and  superior  mesenteric  veins.  The  conti- 
nuity of  these  detached  vessels  with  the  veins  was  not 
actually  observed,  but  it  is  believed  that  they  were  out- 
growths which  had  separated  from  their  veins,  and  that 
they  eventually  unite  with  the  jugular  and  femoral  out- 
growths to  form  a  continuous  lymphatic  system. 

A  third  view,  that  of  Sala  and,  more  recently,  of  Hunt- 
ington and  McClure,  denies  the  primary  connection  of  the 
lymphatics  with  the  veins,  and  maintains  that  they  are 
formed  by  the  fusion  of  mesenchymal  spaces  and  only  sec- 
ondarily make  connections  with  the  veins.  Huntington  and 
McClure  find  the  first  traces  of  the  system  in  cat  embryos  in 
a  series  of  disconnected  spaces  in  the  tissue  immediately  sur- 
rounding the  jugular  veins,  and  believe  that  these  unite  to 
form  the  anterior  lymph  hearts  which  subsequently  become 
connected  with  the  junction  of  the  subclavian  and  jugular 
veins.  Similar  spaces  occur  in  the  tissue  surrounding  other 
veins,  and  the  various  spaces  eventually  unite  together  to 
form  the  lymphatic  system,  and  may  make  connections  with 
veins  other  than  the  subclavian,  as,  for  instance,  the  azygos. 
It  is  worthy  of  note  that  the  spaces  develop  most  rapidly 
where  they  are  in  association  with  regressive  veins,  and 
since  it  is  in  general  the  veins  of  the  left  side  of  the  body 
which  undergo  the  greatest  amount  of  regression,  an  expla- 
nation is  afforded  of  the  situation  of  the  thoracic  duct  on 
that  side. 

Further  observations  are  necessary  to  clear  up  the  dis- 


DEVELOPMENT    OF    THE    LYMPHATIC 'SYSTEM. 


289 


crepancies  in  these  different  views  and  to  determine  the 
essential  point  whether  the  lymphatic  vessels  arise  as  out- 
growths from  the  veins  or  by  the  fusion  of  mesenchymatous 
spaces. 

Lymph  nodes  have  not  been  observed  in  human  embryos 
until  toward  the  end  of  the  third  month  of  development,  but 
they  appear  in  pig  embryos  of  3  cm.  Their  unit  of  struc- 
ture is  a  blood-vessel,  breaking  up 
at  its  termination  into  a  leash  of 
capillaries,  around  which  a  con- 
densation of  lymphocytes  occurs 
in  the  mesenchyme.  A  structure 
of  this  kind  forms  what  is  termed 
a  lymphoid  follicle  and  may  exist, 
even  in  this  simple  condition,  in  the 
adult.  More  frequently,  however, 
there  are  associated  with  the  follicle 
lymphatic  vessels,  or  rather  the 
follicle  develops  in  a  network  of 
lymphatic  vessels,  which  become 
an  investment  of  the  follicle  and 
form  with  it  a  simple  lymph  node. 
This  condition  is,  however,  in 
many  cases  but  transitory,  the 
artery  branching  and  collections  of 
lymphoid  tissue  forming  around 
each  of  the  branches,  so  that  a  series  of  follicles  are  formed, 
which,  together  with  the  surrounding  lymphatic  vessels,  be- 
come enclosed  by  a  connective-tissue  capsule  to  form  a  com- 
pound lymph  node.  Later  trabeculse  of  connective  tissue 
extend  from  the  capsule  toward  the  center  of  the  node, 
between  the  follicles,  the  lymphatic  network  gives  rise  to 
peripheral  and  central  lymph  sinuses,  and  the  follicles,  each 

with  its  arterial  branch,  constitute  the  peripheral  nodules 
26 


Fig.  159. — Diagram  of  a 
Primary  Lymph  Node  of 
AN  Embryo  Pig  of  8  cm. 

a,  artery ;  aid,  afferent  lymph 
duct;  eld,  efferent  lymph 
duct;  f,  follicle. —  (Sabin.) 


290 


DEVELOPMENT    OF    THE   LYMPHATIC    SYSTEM. 


and  the  medullary  cords,  the  portions  of  these  immediately 
surrounding  the  leash  of  capillaries  into  which  the  artery 
dissolves  constituting  the  so-called  germ  centers  in  which 
multiplication  of  the  lymphocytes  occurs. 

In  various  portions  of  the  body,  but  especially  along  the 
root. of  the  mesentery,  what  are  termed  hcemolymph  nodes 


Fig.    160. — Developing   H^molymph   Node. 

be,  central  blood-vessel;  bh,  blood-vessel  at  hilus;  ps,  peripheral  blood 

sinus. —  {Sabin  from  Morris'  Human  Anatomy.) 

occur.  In  these  the  lymph  sinus  is  replaced  by  a  blood 
sinus,  but  with  this  exception  their  structure  resembles  that 
of  an  ordinary  lymph  node,  a  simple  one  consisting  of  a 
follicle,  composed  of  adenoid  tissue  with  a  central  blood 
vessel,  and  a  peripheral  blood  sinus  (Fig.  160). 

The    Development    of    the    Spleen. — Recent    studies 


DEVELOPMENT    OF    THE    SPLEEN.  29 1 

(Mall)  have  shown  that  the  spleen  may  well  be  regarded 
as  possessing  a  structure  comparable  to  that  of  the  lymph 
nodes,  the  pulp  being  more  or  less  distinctly  divided  by 
trabeculae  into  areas  termed  pulp  cords,  the  axis  of  each 
of  which  is  occupied  by  a  twig  of  the  splenic  artery.  The 
spleen,  therefore,  seems  to  fall  into  the  same  category  of 
organs  as  the  lymph  and  heemolymph  nodes,  differing  from 
these  chiefly  in  the  absence  of  sinuses.  It  has  generally 
been  regarded  as  a  development  of  the  mesenchyme  situated 
between  the  two  layers  of  the  mesogastrium.  To  this  view, 
however,  recent  observers  have  taken  exception,  holding 
that  the  tiltimate  origin  of  the  organ  is  in  part  or  entirely 
from  the  coelomic  epithelium  of  the  left  layer  of  the  meso- 
gastrium. The  first  indication  of  the  spleen  has  been  ob- 
served in  embryos  of  the  fifth  week  as  a  slight  elevation  on 
the  left  (dorsal)  surface  of  the  mesogastrium,  due  to  a  local 
thickening  and  vascularization  of  the  mesenchyme,  accom- 
panied by  a  thickening  of  the  coelomic  epithelium  which 
covers  the  elevation.  The  mesenchyme  thickening  presents 
no  differences  from  the  neighboring  mesenchyme,  but  the 
epithelium  is  not  distinctly  separated  from  it  over  its  entire 
surface,  as  it  is  elsewhere  in  the  mesentery.  In  later  stages, 
which  have  been  observed  in  detail  in  pig  and  other  amniote 
embryos,  cells  separate  from  the  deeper  layers  of  the  epi- 
thelium (Fig.  161)  and  pass  into  the  mesenchyme  thicken- 
ing, whose  tissue  soon  assumes  a  different  appearance  from 
the  surrounding  mesenchyme  by  its  cells  being  much 
crowded.  ,  This  migration  soon  ceases,  however,  and  in 
embryos  of  forty-two  days  the  coelomic  epithelium  covering 
the  thickening  is  reduced  to  a  simple  layer  of  cells. 

The  later  stages  of  development  consist  of  an  enlarge- 
ment of  the  thickening  and  its  gradual  constriction  from 
the  surface  of  the  mesogastrium,  until  it  is  finally  united 
to  it  only  by  a  narrow  band  through  which  the  large  splenic 


292  DEVELOPMENT    OF    THE    SPLEEN. 

vessels  gain  access  to  the  organ.     The  cells  differentiate 

themselves  into  trabeculje  and  pulp  cords,  special  collections 

of  lymphoid  cells  around  the  branches  of  the  splenic  artery 

forming  the  Malpighian  corpuscles. 

It  has  already  been  pointed  out  (p.  238)  that  during  embry- 
onic life  the  spleen  is  an  important  hsematopoietic  organ,  both 
red  and  white  corpuscles  undergoing  active  formation  within 
its  substance.  The  Malpighian  corpuscles  are  collections  of 
lymphocytes  in  which  multiplication  takes  place,  and  while 
nothing  is  as  yet  known  as  to  the  fate  of  the  cells  which  are 
contributed  to  the  spleen  from  the  coelomic  epithelium,  since 
they  quickly  come  to  resemble  the  mesenchyme  cells  with  which 
they  are  associated,  yet  the  growing  number  of  observations 


Fig.  161. — Section  through  the  Left  Layer  of  the  Mesogastrium 
OF  A  Chick  Embryo  of  Ninety-three  Hours,  showing  the 
Origin  of  the  Spleen. 

ep,   Ccelomic   epithelium;    ms,   mesenchyme. —  (Tonkoff.) 

indicating  an  epithelial  origin  for  lymphocytes  suggests  the 
possibility  that  the  cells  in  question  may  be  responsible  for  the 
first  leukocytes  of  the  spleen. 

The  Coccygeal  or  Lnschka's  Ganglion. — In  embryos  of 
about  15  cm.  there  is  to  be  found  on  the  ventral  surface  of 
the  apex  of  the  coccyx  a  small  oval  group  of  polygonal  cells, 
clearly  separated  from  the  surrounding  tissue  by  a  mesen- 
chymal capsule.  Later,  connective-tissue  trabeculse  make 
their  way  into  the  mass,  which  thus  becomes  divided  into 
lobules,  and,  at  the  same  time,  a  rich  vascular  supply,  de- 
rived principally  from  branches  of  the  middle  sacral  artery. 


LITERATURE.  293 

penetrates  the  body,  which  thus  assumes  the  adult  condition 
in  which  it  presents  a  general  resemblance  to  a  group  of 
lymph  follicles. 

It  has  generally  been  supposed  that  the  coccygeal  gan- 
glion was  in  part  derived  from  the  sympathetic  nervous 
system  and  belonged  to  the  same  group  of  organs  as  the 
suprarenal  bodies.  The  most  recent  work  on  its  develop- 
ment (Stoerk)  tends,  however,  to  disprove  this  view,  and 
the  ganglion  seems  accordingly  to  find  its  place  among  the 
lymphoid  organs. 

LITERATURE. 

E.  VAN  Beneden  and  C.  Julin  :  "  Recherches  sur  la  formation  des  an- 

nexes fcetales  chez  les  mammiferes,"  Archives  de  Biolog.,  v,  1884. 
A.     C.     Bernays  :     "  Entwickelungsgeschichte     der     Atrioventricular- 

klappen,"  Morphol.  Jahrbuch,  11,  1876. 
l/G.    Born  :    "  Beitrage   zur   Entwicklungsgeschichte   des    Saugethierher- 

zens,"  Archiv  fiir  mikrosk.  Anat,  xxxiii,  1889. 
J.  DissE :  "  Die  Entstehung  des  Blutes  und  der  ersten  Gefasse  im  Huh- 

ntvei/'Archiv  fiir  mikrosk.  Anat.;  xvi,  1879. 
A.  C.  F.  Eternod  :  "  Premiers  stades  de  la  circulation  sanguine  dans 

I'oeuf  et  I'embryon  humain,"  Anat.  Anseiger,  xv,  1899. 
^  W.    His  :    "  Anatomic   menschlicher    Embryonen,"    Leipzig,    1880-1882. 

F.  Hochstetter  :  "  Ueber  die  ursprungliche  Hauptschlagader  der  hin- 

teren  Gliedmasse  des  Menschen  und  der  Saugethiere,  nebst  Bemer- 

kungen  iiber  die  Eintwicklung  der  Endaste  der  Aorta  abdominalis," 

Morphol.  Jahrbuch,  xvi,   i8go. 
F.    Hochstetter  :    "  Ueber   die    Entwicklung   der   A.    vertebralis    beim 

Kaninchen,    nebst    Bemerkungen    iiber    die    Entstehung    der    Ansa 

Vieusseni,"  Morphol.  Jahrbuch,  xvi,  1890. 
F.  Hochstetter  :  "  Beitrage  zur  Entwicklungsgeschichte  des  Venensys- 

tems  der  Amnioten,"  Morphol.  Jahrbuch,  xx,  1893. 
^'  W.  H.  Howell  :   "  The  Life-history  of  the   Formed  Elements  of  the 

Blood,  Especially  the  Red  Blood-corpuscles,"  Journ.  of  Morphol., 

IV,   1890. 
W.    H.    Howell  :    "  Observations    on    the   Occurrence,    Structure,    and 

Function   of  the   Giant-cells   of  the    Marrow,"  Journ.   of  Morph., 

rv,  1890. 
n/  G.  H.  Huntington  and  C.  F.  W.  McClure:  "Development  of  Post- 

cava  and  Tributaries   in  the  Domestic   Cat,"  Amcr.  Journ.   Anat.. 

VI,  1907. 


294  LITERATURE, 

^  G.  H.  Huntington  and  C.  F.  McClure:  "The  Development  of  the 

Main  Lymph  Channels  of  the  Cat  in  their  Relations  to  the  Venous 

System,"  Amer.  Journ.  Anat.,  vi,  1907. 
V    C.  A.  Kling  :  "  Studien  iiber  die  Entwicklung  der  Lymphdriisen  beim 

Menschen,"  Archiv.  fur  mikrosk.  Anat.,  lxiii^  1904. 
H.  Lehmann  :  "  On  the  Embryonic  History  of  the  Aortic  Arches  in 

Mammals,"  Anat.  Anseiger,- :x.x\i,  1905.  "^ 

V-    F.  T.  Lewis  :  "  The  Development  of  the  Vena  Cava  Inferior,"  Amer. 

Journ.  of  Anat.,  1,  1902. 
''  F.  T.  Lewis  :  "  The  Development  of  the  Veins  in  the  Limbs  of  Rabbit 

Embryos,"  Amer.  Journ.  Anat.,  v,  1906. 
''   F.  T.  Lewis  :  "  The  Development  of  the  Lymphatic  System  in  Rabbits," 

Amer.  Journ.  Anat.,  v,  1906. 
W.  A.  LocY :  "  The  Fifth  and  Sixth  Aortic  Arches  in  Chick  Embryos, 

with   Comments   on  the  Condition  of  the  same  Vessels  in  other 

Vertebrates,"  Anat.  Anzeiger,  xxix,  1906. 
F.  P.  Mall  :  "  Development  of  the  Internal  Mammary  and  Deep  Epi- 
gastric Arteries  in  Man,"  Johns  Hopkins  Hospital  Bulletin,  1898. 
^F.  P.  Mall:  "On  the  Development  of  the  Blood-vessels  of  the  Brain 

in  the  Human  Embryo,"  Amer.  Journ.  Anat.,  iv,  1905. 
C.  S.  Minot:  "On  a  Hitherto  Unrecognized  Form  of  Blood  Circula- 
tion without  Capillaries  in  the  Organs  of  Vertebrata,"  Proc.  Bos- 
ton Soc.  Nat.  Hist.,  xxix,  1900. 
E.  Retterer  :  "  Sur  la  part  que  prend  I'epithelium  a  la  formation  de  la 

bourse    de    Fabricius,    des    amygdales    et    des    plaques    de    Peyer," 

Journ.  de  I'Anat.  et  de  la  Physiol.,  xxix,  1893. 
C.  Rose:  "Zur  Entwicklungsgeschichte  des  Saugethierherzens,"  Mor- 

phol.  Jahrhuch,  xv,  1889. 
V  Florence  R.  Sabin  :  "  On  the  Origin  of  the  Lymphatic  System  from 

the  Veins  and  the  Development  of  the  Lymph  Hearts  and  Thoracic 

Duct  in  the  Pig,"  Amer.  Journ.  of  Anat.,  i,  1902. 
Florence  R.  Sabin  :  "  The  Development  of  the  Lymphatic  Nodes  in 

the  Pig  and  their  Relation  to  the  Lymph  Hearts,"  Amer.  Journ. 

Anat.,  IV,  1905. 
P.  Stohr  :  "  Ueber  die  Entwicklung  der  Darmlymphknotchen  und  iiber 

die   Riickbildung   von   Darmdriisen,"   Archiv  fUr  mikrosk.    Anat., 

li,  1898. 
O.  VAN  DER  Stricht  :  "  Nouvelles  recherches  sur  la  genese  des  globules 

rouges  et  des  globules  blancs  du  sang,"  Archives  de  Biolog.,  xii, 

1892. 
O.  VAN  DER  Stricht  :  "  De  la  premiere  origine  du  sang  et  des  capillaires 

sanguins  dans  I'aire  vasculaire  du  Lapin,"  Comptes  Rendus  de  la 

Soc.  de  Biolog.  Paris,  Ser.   10,  11,  1895. 


LITERATURE,  295 

O.    Stoerk  :    "  Ueber  die   Chromreaktion   der   Glandula   coccygea   und 

die  Beziehung  dieser  Driise  zum  Nervus  sympathicus,"  Arch,  fur 

mikroskop.  Anat.,  lxix,  1906. 
J.   Tandler:   "Zur  Entwicklungsgeschichte  der  Kopfarterien  bei   den 

Mammalia,"  Morphol.  Jahrbuch,  xxx,  1902. 
J.  Tandler:   "Zur  Entwickelungsgeschichte   der  menschlichen  Darm- 

arterien,"  Anat.  Hefte,  xxiii,  1903. 
^'^.  ToNKOFF :  "  Die  Entwickelung  der  Milz  bei  den  Amnioten,"  Arch. 

fur  mikrosk.  Anat.,  lvi,  1900. 
Bertha  de  Vriese:  "Recherches  sur  revolution  des  vaissaux  sanguins 

des  membres  chez  I'homme,"  Archives  de  Biolog.,  xviii,  1902. 


CHAPTER  X. 

THE    DEVELOPMENT    OF    THE    DIGESTIVE 
TRACT   AND    GLANDS. 

The  greatest  portion  of  the  digestive  tract  is  formed  by 
the  constriction  off  of  the  dorsal  portion  of  the  yolk-sac, 
as  shown  in  Fig.  39,  the  result  being  the  formation  of  a 
cylinder,  closed  at  either  end,  and  composed  of  a  layer  of 
splanchnic  mesoderm  lined  on  its  inner  surface  by  endo- 
derm.  This  cylinder  is  termed  the  archenteron  and  has 
connected  with  it  the  yolk-stalk  and  the  allantois,  the  latter 
communicating  with  its  somewhat  dilated  terminal  portion, 
which  also  receives  the  ducts  of  the  primitive  kidneys  and 
is  known  as  the  cloaca  (Fig.  163). 

At  a  very  early  stage  of  development  the  anterior  end 
of  the  embryo  begins  to  project  slightly  in  front  of  the 
yolk-sac,  so  that  a  shallow  depression  is  formed  between 
the  two  structures.  As  the  constriction  of  the  embryo  from 
the  sac  proceeds,  the  anterior  portion  of  the  brain  becomes 
bent  ventrally  and  the  heart  makes  its  appearance  immedi- 
ately in  front  of  the  anterior  surface  of  the  yolk-sac,  and 
so  the  depression  mentioned  above  becomes  deepened  (Fig. 
162)  to  form  the  oral  sinus.  The  floor  of  this,  lined  by 
ectoderm,  is  immediately  opposite  the  anterior  end  of  the 
archenteron,  and,  since  mesoderm  does  not  develop  in  this 
region,  the  ectoderm  of  the  sinus  and  the  endoderm  of  the 
archenteron  are  directly  in  contact,  forming  a  thin  pharyn- 
geal membrane  separating  the  two  cavities  (Fig.  162,  pm). 
In  embryos  of  2.15  mm.  this  membrane  is  still  existent,  but 
soon  after  it  becomes  perforated  and  finally  disappears,  so 
that  the  archenteron  and  oral  sinus  become  continuous. 

296 


DEVELOPMENT    OF    THE    DIGESTIVE    TRACT. 


297 


Toward  its  posterior  end  the  archenteron  comes  into 
somewhat  similar  relations  with  the  ectoderm,  though  a 
marked  difference  is  noticeable  in  that  the  area  over  which 
the  cloacal  endoderm  is  in  contact  with  the  ectoderm  to  form 
the  cloacal  membrane  (Fig.  163,  cm)  lies  a  little  in  front 
of  the  actual  end  of  the  archenteric  cylinder,  the  portion  of 
the  latter  which  lies  posterior  to  the  membrane  forming 
what  has  been  termed 
the  post-anal  gut  (p.an). 
This  diminishes  in  size 
during  development  and 
early  disappears  alto- 
gether, and  the  pouch-like 
fold  seen  in  Fig.  163  be- 
t\yeen  the  intestinal  por- 
tion of  the  archenteron 
■  and  the  allantoic  stalk 
(al)  deepening  until  its 
floor  comes  into  contact 
with  the  cloacai  mem- 
brane, the  cloaca  becomes 
divided  into  a  ventral  por- 
tion, with  which  the  allan- 
tois  and  the  primitive  ex- 
cretory ducts  (zu)  are 
connected,  and  a  dorsal  portion  which  becomes  the  lower 
end  of  the  rectum.  This  latter  abuts  upon  the  dorsal  por- 
tion of  the  cloacal  membrane,  and  this  eventually  ruptures, 
so  that  the  posterior  communication  of  the  archenteron  with 
the  exterior  becomes  established.  This  rupture,  however, 
does  not  occur  until  a  comparatively  late  period  of  develop- 
ment, until  after  the  embryo  has  reached  the  fetal  stage; 
nor  does  the  position  of  the  membrane  correspond  with  the 
adult  anus,  since  later  there  is  a  considerable  development 


Fig.  162.— Reconstruction  of  the 
Anterior  Portion  of  an   Embryo 

OF   2.15    MM. 

ab,  Aortic  bulb ;  h,  heart ;  0,  auditory 
capsule ;  op,  optic  evagination ;  pm, 
pharyngeal    membrane. —  {His.) 


298 


DIGESTIVE    TRACT    AND    GLANDS. 


of  mesoderm  around  the  mouth  of  the  cloaca,  bulging  out, 
as  it  were,  the  surrounding  ectoderm,  more  especially  an- 
teriorly where  it  forms  the  large  genital  tubercle  (see 
Chapter  XIII),  and  posteriorly  when  it  produces  the  anal 
tubercle.  This  appears  as  a  rounded  elevation  on  each  side 
of  the  median  line,  immediately  behind  the  clpacal  mem- 
brane and  separated  from  the  root  of  the  caudal  projec- 
tion by  a  depression,  the  precaudal  recess.     Later  the  two 


<       }f^     w- 


nc 


Fig.  163. — Reconstruction  of  the  Hind  End  of  an  Embryo  6.5  mm. 

Long. 

al,  Allantois;  h,  belly-stalk;  cl,  cloaca;  cm,  cloacal  membrane;  i,  intes- 
tine ;  n,  spinal  cord ;  nc,  notochord ;  p.an,  post-anal  gut ;  ur,  out- 
growth to  form  ureter  and  metanephros ;  w,  Wolffian  duct. — 
{Keihel.') 


elevations  unite  across  the  median  line  to  form  a  transverse 
ridge,  the  ends  of  which  curve  forward  and  eventually  meet 
in  front  of  the  original  anal  orifice.  From  the  mesoderm 
of  the  circular  elevation  thus  produced  the  external  sphincter 
ani  muscle  is  formed,  and  it  would  seem  that  so  much  of  the 


DEVELOPMENT    OF    THE    MOUTH    REGION,  299 

lower  end  of  the  rectum  as  corresponds  to  this  muscle  is 
formed  by  the  inner  surface  of  the  elevation  and  is  there- 
fore ectodermal.  The  definitive  anus  being  at  the  end  of 
this  terminal  portion  of  the  gut  is  therefore  some  distance 
away  from  the  position  of  the  original  cloacal  membrane. 

It  will  be  noticed  that  the  digestive  tract  thus  formed 
consists  of  three  distinct  portions,  an  anterior,  short,  ecto- 
dermal portion,  an  endodermal  portion  representing  the 
original  archenteron,  and  a  posterior  short  portion  which 
is  also  ectodermal.  The  differentiation  of  the  tract  into 
its  various  regions  and  the  formation  of  the  various  organs 
found  in  relation  with  these  may  now  be  considered. 

The  Development  of  the  Mouth  Region. — The  deep- 
ening of  the  oral  sinus  by  the  development  of  the  first  bran- 
chial arch  and  its  separation  into  the  oral  and  nasal  cavities 
by  the  development  of  the  palate  have  already  been  described 
(p.  88),  but,  for  the  sake  of  continuity  in  description,  the 
latter  process  may  be  briefly  recalled.  At  first  the  nasal 
pits  communicate  with  the  oral  sinus  by  grooves  lying  one 
on  each  side  of  the  fronto-nasal  process,  but  by  the  union 
of  the  latter  with  the  maxillary  process  this  communication 
is  interrupted  and  the  pits  make  new  connections  with  the 
oral  sinus  behind  the  maxillary  process.  At  about  the  fifth 
week  a  downgrowth  of  epithelium  into  the  substance  of 
both  the  maxillary  and  fronto-nasal  processes  above  and  the 
mandibular  process  below,  takes  place  and  the  surface  of 
the  downgrowth  becomes  marked  by  a  deepening  groove 
(Fig,  164),  which  separates  an  anterior  fold,  the  lip,  from 
the  jaw  proper  (Fig.  165),  From  the  maxillo-palatine 
portions  of  the  upper  jaw,  shelf -like  ridges  then  begin  to 
grow  at  first  downwards  and  then  medially,  and  at  about 
the  beginning  of  the  third  month  these  meet  in  the  median 
line  to  form  the  palate,  and  unite  anteriorly  with  the  pre- 


300  DEVELOPMENT    OF    THE    MOUTH    REGION. 

maxillae,  thus  completing  the  separation  of  the  definitive 
mouth  from  the  nasal  cavity.  At  the  point  of  meeting  of 
the  shelves  with  the  premaxillEe  a  small  communication  be- 
tween the  two  cavities  persists  for  a  time,  frequently  until 
after  birth ;  it  allows  passage  of  the  anterior  palatine  vessels 
and  nerves,  and  places  the  organ  of  Jacobson  (p.  459)  in 
communication  with  the  mouth.  Later  the  opening  becomes 
closed  over  by  mucous  membrane,  but  it  may  be  recognized 


Fig.  164. — View  of  the  Roof  of  the  Oral  Fossa  of  Embryo  showing 
THE  Lip-groove  and  the  Formation  of  the  Palate.— (if w.) 

in  the  dried  skull  as  the  foramen  incisivum  (anterior  pala- 
tine canal). 

When  the  ridges  which  become  the  palatal  plates  are  first 
formed  they  have  an  almost  vertical  direction,  projecting  down- 
ward and  somewhat  inward  between  the  sides  of  the  tongue 
and  the  alveolar  processes.  The  tongue  at  this  stage  almost 
completely  fills  the  oral  cavity,  its  dorsum  being  in  contact 
with  the  base  of  the  skull.  Later,  the  lower  jaw,  which  at  first 
is  considerably  shorter  than  the  upper  one,  increases  in  length 
and  at  the  same  time  also  in  breadth,  and  also  assumes  a  more 
horizontal  position.  The  tongue  then  sinks  down  between  the 
Meckelian  cartilages  and  the  palatal  ridges  bend  dorsally  so 
as  to  become  horizontal,  and  their  further  growth  leads  to 
their  union  in  the  median  line  to  form  the  roof  of  the  mouth. 

Occasionally  there  is  a  failure  of  the  union  of  the  palatal 
plates,  the  condition  known  as  cleft  palate  resulting.     The  in- 


DEVELOPMENT    OF    THE    MOUTH    REGION.  30I 

hibition  of  development  which  brings  about  this  condition  may 
take  place  at  different  stages,  but  frequently  it  occurs  while 
the  plates  still  have  an  almost  vertical  direction.  A  second 
variety  of  cleft  palates  may  result  from  failure  of  the  palatal 
plates  to  unite  with  the  premaxillary  portions  of  the  jaw,  a 
condition  which  is  associated  with  hare-lip  (see  page  89), 
although  this  abnormality  may  exist  without  an  involvement 
of  the  palate. 

Before  the  formation  of  the  palate  begins,  a  pouch  is 
formed  in  the  median  line  of  the  roof  of  the  oral  sinus,  just 
in  front  of  the  pharyngeal  membrane,  by  an  outgrowth  of 
the  epithelium.  This  pouch,  known  as.  Rathke's  pouch, 
comes  in  contact  above  with  a  downgrowth  from  the  floor 
of  the  brain  and  forms  with  it  the  pituitary  body  (see  p.  424). 

The  Development  of  the  Teeth. — When  the  epithelial 
downgrowth  which  gives  rise  to  the  lip  groove  is  formed, 
a  horizontal  outgrowth  develops  from  it  which  extends  back- 
ward into  the  substance  of  the  jaw,  forming  what  is  termed 
the  dental  shelf  (Fig.  165,  A),  This  at  first  is  situated  on 
the  anterior  surface  of  the  jaw,  but  with  the  continued  devel- 
opment of  the  lip  fold  it  is  gradually  shifted  until  it  comes 
to  lie  upon  the  free  surface  (Fig.  165,  B),  where  its  super- 
ficial edge  is  marked  by  a  distinct  groove,  the  dental  groove 
(Fig.  164).  At  first  the  dental  shelf  of  each  jaw  is  a  con- 
tinuous plate  of  cells,  uniform  in  thickness  throughout  its 
entire  width,  but  later  ten  thickenings  develop  upon  its  deep 
edge,  and  beneath  each  of  these  the  mesoderm  condenses  to 
form  a  dental  papilla,  over  the  surface  of  which  the  thick- 
ening moulds  itself  to  form  a  cap,  termed  the  enamel  organ 
(Fig.  165,  B).  These  ten  papill3e4n -each  jaw,  with  their 
enamel  caps,  represent  the  teeth  of  the  first  dentition. 

The  papillae  do  not,  however,  project  into  the  very  edge 
of  the  dental  shelf,  but  obliquely  into  what,  in  the  lower 
jaw,  was  originally  its  under  surface  (Fig.  165,  B),  so  that 
the  edge  of  the  shelf  is  free  to  grow  still  deeper  into  the 


302 


DEVELOPMENT    OF    THE    TEETH. 


surface  of  the  jaw.  This  it  does,  and  upon  the  extension 
so  formed  there  is  developed  in  each  jaw  a  second  set  of 
thickenings,  beneath  each  of  which  a  dental  papilla  again 
appears.  These  tooth-germs  represent  the  incisors,  canines, 
and  premolars  of  the  permanent  dentition.  The  lateral 
edges  of  the  dental  shelf  being  continued  outward  toward 
the  articulations  of  the  jaws  as  prolongations  which  are  not 


Fig.  165. — Transverse  Sections  through  the  Lower  Jaw  showing 
THE  Formation  of  the  Dental  Shelf  in  Embryos  of  (A)  17  mm. 
AND  (B)  40  MM. —  (Rose.) 

connected  with  the  surface  epithelium,  opportunity  is 
afforded  for  the  development  of  three  additional  thicken- 
ings on  each  side  in  each  jaw,  and,  papillae  developing 
beneath  these,  twelve  additional  tooth-germs  are  formed. 
These  represent  the  permanent  molars;  their  formation  is 
much  later  than  that  of  the  other  teeth,  the  germ  of  the 
second  molar  not  appearing  until  about  the  sixth  week  after 


DEVELOPMENT    OF    THE    TEETH.  3O3 

birth,  while  that  of  the  third  is  delayed  until  about  the  fifth 
year. 

As  the  tooth-germs  increase  in  size,  they  approach  nearer 
and  nearer  to  the  surface  of  the  jaw,  and  at  the  same  time 
the  enamel  organs  separate  from  the  dental  shelf  until  their 
connection  with  it  is  a  mere  neck  of  epithelial  cells.  In  the 
meantime  the  dental  shelf  itself  has  been  undergoing-  degen- 
eration and  is  reduced  to  a  reticulum  which  eventually  com- 
pletely disappears,  though  fragments  of  it  may  occasionally 
persist  and  give  rise  to  various  malformations.  With  the 
disappearance  of  the  last  remains  of  the  shelf,  the  various 
tooth-germs  naturally  lose  all  connection  with  one  another. 

It  will  be  seen,  from  what  has  been  said,  that  each  tooth- 
germ  consists  of  two  portions,  one  of  which,  the  enamel 
organ,  is  derived  from  the  ectoderm,  while  the  other,  the 
dental  papilla,  is  mesenchymatous.  Each  of  these  gives 
rise  to  a  definite  portion  of  the  fully  formed  tooth,  the 
enamel  organ,  as  its  name  indicates,  producing  the  enamel, 
while  from  the  dental  papilla  the  dentine  and  pulp  are  formed. 

The  cells  of  the  enamel  organ  which  are  in  contact  with 
the  surface  of  the  papilla,  at  an  early  stage  assume  a  cylin- 
drical form  and  become  arranged  in  a  definite  layer,  the 
enamel  membrane  (Fig.  i66,  SEi),  while  the  remaining 
cells  (SEa)  apparently  degenerate  eventually,  though  they 
persist  for  a  time  to  form  what  has  been  termed  the  enamel 
pulp.  The  formation  of  the  enamel  seems  to  be  due  to  the 
direct  transformation  of  the  enamel  cells,  the  process  begin- 
ning at  the  basal  portion  of  each  cell,  and  as  a  result,  the 
enamel  consists  of  a  series  of  prisms,  each  of  which  repre- 
sents one  of  the  cells  of  the  enamel  membrane.  The  trans- 
formation proceeds  until  the  cells  have  become  completely 
converted  into  enamel  prisms,  except  at  their  very  tips, 
which  form  a  thin  membrane,  the  enamel  cuticle,  which  is 
shed  soon  after  the  eruption  of  the  teeth. 


304 


DEVELOPMENT    OF    THE    TEETH. 


The  dental  papillse  are  at  first  composed  of  ,a  closely 
packed  mass  of  mesenchyme  cells,  which  later  become  differ- 
entiated into  connective  tissue  into  which  blood-vessels  and 
nerves  penetrate.     The  superficial  cells  form  a  more  or  less 


Fig.   1 66. — Section    through    the    First    Molar    Tooth    of  a    Rat, 

Twelve  Days  Old. 
Ap,    Periosteum;    E.    dentine;    Ep,    epidermis;    Od,    odontoblasts;    S, 

enamel ;  SEa  and  SEi,  outer  and  inner  layers  of  the  enamel  organ  ; 

SE,  portion  of  the  enamel  organ  which  does  not  produce  enamel. — 

(von  Brunn.) 


definite    layer    (Fig.    i66,    od),    and    are    termed    odonto- 
blasts, having-  the  function  of  manufacturing-  the  dentine. 


DEVELOPMENT    OF    THE    TEETH.  30^ 

This  they  accompHsh  in  the  same  manner  as  that  in  which 
the  periosteal  osteoblasts  produce  bone,  depositing  the  den- 
tine between  their  surfaces  and  the  adjacent  surface  of  the 
enamel.  The  outer  surface  of  each  odontoblast  is  drawn 
out  into  a  number  of  exceedingly  fine  processes  which  ex- 
tend into  the  dentine  to  occupy  the  minute  dentinal  tubules, 
just  as  processes  of  the  osteoblasts  occupy  the  canaliculi  of 
bone. 

At  an  early  stage  the  enamel  membrane  forms  an  almost 
complete  investment  for  the  dental  papilla  (Fig.  i66),  but, 
as  the  ossification  of  the  tooth  proceeds,  it  recedes  from  the 
lower  part,  until  finally  it  is  confined  entirely  to  the  crown. 
The  dentine  forming  the  roots  of  the  tooth  then  becomes 
enclosed  in  a  layer  of  cement,  which  is  true  bone  and  serves 
to  unite  the  tooth  firmly  to  the  walls  of  its  socket.  As  the 
tooth  increases  in  size,  its  extremity  is  brought  nearer  to 
the  surface  of  the  gum  and  eventually  breaks  through,  the 
eruption  of  the  first  teeth  usually  taking  place  during  the 
last  half  of  the  first  year  after  birth.  The  growth  of  the 
permanent  teeth  proceeds  slowly  at  first,  but  later  it  becomes 
more  rapid  and  produces  pressure  upon  the  roots  of  the 
primary  teeth.  These  roots  then  undergo  partial  absorp- 
tion, and  the  teeth  are  thus  loosened  in  their  sockets  and  are 
readily  pushed  out  by  the  further  growth  of  the  permanent 
teeth. 

The  dates  and  order  of  the  eruption  of  the  teeth  are  subject 
to  considerable  variation,  but  the  usual  sequence  is  somewhat 
as  follows : 

Primary  Dentition. 

Median  incisors,  6th  to  8th  month. 

Lateral  incisors,   7th   to  Qth  month. 

First  molars,   Beginning  of  2d  year. 

Canines, i4  years. 

Second  molars,  3  to  si  years. 

27 


306  DEVELOPMENT    OF   THE   TONGUE, 

The  teeth  of  the  lower  jaw  generally  precede  those  of  the 
upper. 

Permanent  Dentition. 

First  molars,  7th  year. 

Middle  incisors,   8th  year. 

Lateral  incisors,   gth  year. 

First  premolars,   loth  year. 

Second  premolars,   nth  year. 

Canines,  )  ,,    ,        ,,, 

,  '      ,  I    13th  to  14th  years. 

Second  molars,  j 

Third  molars,    17th  to  40th  years. 

In  a  considerable  percentage  of  individuals  the  third  molars 
(wisdom  teeth)  never  break  through  the  gums,  and  frequently 
when  they  do  so  they  fail  to  reach  the  level  of  the  other  teeth, 
and  so  are  only  partly  functional.  These  and  other  peculiari- 
ties of  a  structural  nature  shown  by  these  teeth  indicate  that 
they  are  undergoing  a  retrogressive  evolution. 

The  Development  of  the  Tongue. — Strictly  speaking, 
the  tongue  is  largely  a  development  of  the  pharyngeal  re- 
gion of  the  digestive  tract  and  only  secondarily  grows  for- 
v^ard  into  the  floor  of  the  mouth.  In  embryos  of  about  3 
mm.  there  may  be  seen  in  the  median  line  of  the  floor  of 
the  mouth,  between  the  ventral  ends  of  the  first  and  second 
branchial  arches,  a  small  rounded  elevation  which  has  been 
termed  the  tuberculum  impar  (Fig.  171,  t).  In  later  stages 
(Fig.  167,  A)  this  becomes  larger  and  reaches  its  greatest 
development  in  embryos  of  about  8  mm.,  after  which  it 
becomes  less  prominent  and  finally  unrecognizable;  but 
before  this  there  has  appeared  on  each  side  of  the  floor  of 
the  mouth  a  longitudinal  groove,  each  of  which  at  its  ante- 
rior end  bends  medially  toward  its  fellow.  By  these  alveolo- 
lingual  grooves  an  area  is  marked  out  in  the  floor  of  the 
mouth  which  -gradually  becomes  more  and  more  prominent 
and  rounded  upon  its  oral  surface,  and  forms  the  anterior 
portion  of  the  tongue  (Fig.  167,  B,  t^).  This  median  ele- 
vation is  bounded  at  the  sides  and  almost  to  the  median  line 
in  front  by  the  alveolo-lingual  "grooves,  and  posteriorly  it  is 


DEVELOPMENT    OF   THE   TONGUE. 


307 


separated  from  the  anterior  edge  of  the  second  branchial 
arch  by  a  distinct  V-shaped  groove,  at  the  apex  of  which  is 
a  deep  circular  depression,  the  foramen  ccecum  (see  p.  313). 
The  posterior  portion  of  the  tongue  arises  as  thickenings 
of  the  ventral  ends  of  the  second  branchial  arches,  and  is 
consequently  a  V-shaped  structure,  into  the  angle  of  which 
the  posterior  part  of  the  anterior  portion  of  the  tongue 
fits  (Fig.  168).     The  two  portions,  anterior  and  posterior, 


Fig.  167. — Floor  of  the  Pharynx  of  Embryos  of  (A)  7  and  (5)   10 

MM.,   SHOWING  THE   DEVELOPMENT  OF  THE  ToNGUE. 

Ep,   Epiglottis;   Sp,  prsecervical   sinus;    f   and   t',  median  and   lateral 
portions  of  the  tongue;  /  to  IV,  branchial  arches. — {His.) 


eventually  fuse  together,  but  the  groove  which  originally, 
separated  them  remains  more  or  less  clearly  distinguishable, 
the  vallate  papillae  (see  p.  460)  developing  immediately  an- 
terior to  it. 

The  tongue  is  essentially  a  muscular  organ,  being  formed 
of  a  central  mass  of  muscular  tissue,  enclosed  at  the  sides  and 
dorsally  by  mucous  membrane  derived  from  the  floor  of  the 
mouth  and  pharynx.  The  muscular  tissue  consists  partly  of 
fibers  limited  to  the  substance  of  the  tongue  and  forming  the 
w.  lingualis,  and  also  of  a  number  of  extrinsic  muscles,  the 
hyoglossi^  genioglossi,  styloglossi,  glosso palatini,  and  chondro- 


308 


DEVELOPMENT    OF    THE   TONGUE. 


glossi.  The  last  two  muscles  are  innervated  by  the  vagus 
nerve,  and  the  remaining  extrinsic  muscles  receive  fibers  from 
the  hypoglossal,  while  the  lingualis  is  supplied  partly  by  the 
hypoglossal  and  partly,  apparently,  by  the  facial  through  the 
chorda  tympani.  That  the  facial  should  take  part  in  the  sup- 
ply is  what  might  be  expected 
from  the  mode  of  develop- 
ment of  the  tongue,  but  the 
hypoglossal  has  been  seen  to 
correspond  to  certain  pri- 
marily postcranial  metameres 
(p.  179),  and  its  relation  to 
structures  taking  part  in  the 
formation  of  an  organ  belong- 
ing to  the  anterior  part  of  the 
pharynx  seems  somewhat 
anomalous.  It  may  be  sup- 
posed that  in  the  evolution  of 
the  tongue  the  extrinsic  mus- 
cles, together  with  a  certain 
amount  of  the  lingualis,  have 
grown  into  the  tongue  thick- 
enings from  regions  situated 
much  further  back,  for  the  most  part  from  behind  the  last 
branchial  arch. 

Such  an  invasion  of  the  tongue  by  muscles  from  posterior 
segments  would  explain  the  distribution  of  its  sensory  nerves. 
The  anterior  portion,  from  its  position,  would  naturally  be  sup- 
plied by  branches  from  the  fifth  and  seventh  nerves,  while  the 
posterior  portion  might  be  expected  to  be  supplied  by  the 
seventh.  There  seems,  however,  to  have  been  a  dislocation 
forward,  if  it  may  be  so  expressed,  of  the  mucous  membrane, 
the  sensory  distribution  of  the  ninth  nerve  extending  forward 
upon  the  posterior  part  of  the  anterior  portion  of  the  tongue, 
while  a  considerable  amoimt  of  the  posterior  portion  is  supplied 
by  the  tenth  nerve.  The  distribution  of  the  sensory  fibers  of 
the  facial  is  probably  confined  entirely  to  the  anterior  portion, 
though  further  information  is  needed  to  determine  the  exact 
distribution  of  both  the  motor  and  sensory  fibers  of  this  nerve 
in  the  tongue. 

The  Development  of  the  Salivary  Glands. — In  em- 
bryos of  al)out  8  mm.  a  slight  furrow  may  be  observed  in 
the  floor  of  the  groove  which  connects  the  lip  grooves  of 


Fig.  168. — The  Floor  of  the 
Pharynx  of  an  Embryo  of 
ABOUT    20    MM 

ep,  Epiglottis ;  fc,  foramen  caerum ; 
t'^  and  f  median  and  lateral  por- 
tions   of   the   tongue. —  (His.) 


THE  SALIVARY   GLANDS. 


309 


the  upper  and  lower  jaws  at  the  angle  of  the  mouth  and 
may  be  known  as  the  cheek  groove.  In  later  stages  this 
furrow  deepens  and  eventually  becomes  closed  in  to  form  a 
hollow  tubular  structure,  which  in  embryos  of  17  mm.  has 


Fig.  169. — Diagram  of  the  Distribution  of  the  Sensory  Nerves  of 

THE  Tongue. 
The  area  supplied  by  the  fifth    (and  seventh)    nerve   is   indicated  by 

the  transverse  lines ;  that  of  the  ninth  by  the  oblique  lines ;  and  that 

of  the  tenth  by  the  small  circles. —  {Zander.) 


separated  from  the  epithelium  of  the  floor  of  the  cheek 
groove  except  at  its  anterior  end  and  has  become  embedded 
in  the  connective  tissue  of  the  cheek.     This  tube  is  readily 


3IO 


THE    SALIVARY    GLANDS. 


recognizable  as  the  parotid  gland  and  Stenson's  duct,  and 
from  the  latter  as  it  passes  across  the  masseter  muscle  a 
pouch-like  outgrowth  is  early  formed  which  probably  rep- 
resents the  socia  parotidis. 

The  submaxillary  gland  and  Wharton's  duct  appear  in 
embryos  of  about  13  mm.  as  a  longitudinal  ridge-like  thick- 
ening of  the  epithelium  of  the  floor  of  the  alveolo-lingual 
groove  (see  p.  306).  This  ridge  gradually  separates  from 
behind  forward  from  the  floor  of  the  groove  and  sinks  into 
the  subjacent  connective  tissue,  retaining,  however,  its  con- 


0«i/ 


Fig.   170.^ — An  Oblique  Section  through  the  Mouth  Cavity  of  an 

Embryo  of  about  16  to  17  mm. 
cm,  Meckel's  cartilage;  id,  inferior  dental  nerve;  nl,  lingual  nerve;  P, 

parotid  gland;   SL,  septum   of  the  tongue;  si,   sublingual  gland; 

sm,  submaxillary  gland;  t,  tooth;  XII,  hypoglossal  nerve.^(His.) 

nection  with  the  epithelium  at  its  anterior  end,  which  indi- 
cates the  position  of  the  opening  of  the  duct.  In  the  vicinity 
of  this  there  appear  in  embryos  of  24.4  mm.  five  small  bud- 
like downgrowths  of  the  epithelium,  which  later  increase 
considerably  in  number  as  well  as  in  size,  and  constitute 
a  group  of  glands  which  are  generally  spoken  of  as  the 
sublingual  gland. 

As  these  representatives  of  the  various  glands  increase 
in  length,  they  become  lobed  at  their  deeper  ends,  and  the 
lobes  later  give  rise  to  secondary  outgrowths  which  branch 
repeatedly,  the  terminal  branches  becoming  the  alveoli  of 


THE    PHARYNX.  3  I  I 

the  glands.     A  lumen  early  appears  in  the  duct  portions  of 
•  the  structures,  the  alveoli  remaining  solid  for  a  longer  time, 
although  they  eventually  also  become  hollow. 

It  is  to  be  noted  that  each  parotid  and  submaxillary  consists 
of  a  single  primary  outgrowth,  and  is  therefore  a  single  struc- 
ture and  not  a  union  of  a  number  of  originally  separate  parts. 
The  sublingual  glands  of  adult  anatomy  are  usually  described 
as  opening  upon  the  floor  of  the  mouth  by  a  number  of  separate 
ducts.  This  arises  from  the  fact  that  the  majority  of  the 
glands  which  form  in  the  vicinity  of  the  opening  of  Wharton's 
duct  remain  quite  small,  only  one  of  them  on  each  side  giving 
rise  to  the  sublingual  gland  proper.  The  small  glands  have 
been  termed  the  alveolo-lingual  glands,  and  each  one  of  them 
is  equivalent  to  a  parotid  or  submaxillary  gland.  In  other 
words,  there  are  in  reality  not  three  pairs  of  salivary  glands, 
but  from  fourteen  to  sixteen  pairs,  there  being  usually  from 
eleven  to  thirteen  alveolo-lingual  glands  on  each  side. 

The  Development  of  the  Pharynx. — The  phaynx  rep- 
resents the  most  anterior  part  of  the  archenteron,  that  por- 
tion in  which  the  branchial  arches  develop,  and  in  the  embryo 
it  is  relatively  much  longer  than  in  the  adult,  the  diminu- 
tion being  brought  about  by  the  folding  in  of  the  posterior 
arches  and  the  formation  of  the  sinus  prsecervicalis  already 
described  (p.  86).  Between  the  various  branchial  arches, 
grooves  occur,  representing  the  endodermal  portions  of  the 
grooves  which  separate  the  arches.  During  development 
the  first  of  these  becomes  converted  into  the  tympanic  cavity 
of  the  ear  and  the  Eustachian  tube  (see  Chapter  XV)  ;  the 
second  disappears  in  its  upper  part,  the  lower  persisting  as 
the  fossa  in  which  the  tonsil  is  situated;  while  the  lower 
parts  of  the  remaining  two  are  represented  by  the  sinus 
piriformis  of  the  larynx  (His),  and  also  leave  traces  of 
their  existence  in  detached  portions  of  their  epithelium 
which  form  what  are  termed  the  branchial  epithelial  bodies, 
and  take  part  in  the  formation  of  the  thyreoid  and  thymus 
glands. 


312 


THE    PHARYNX. 


In  the  floor  of  the  pharynx  behind  the  thickenings  which 
produce  the  tongue  there  is  to  be  found  in  early  stages 
a  pair  of  thickenings  passing  horizontally  backward  and 
uniting  in  front  so  that  they  resemble  an  inverted  U  (Fig. 
171,  /).  These  ridges,  which  form  what  is  termed  the 
fttrcula  (His),  are  concerned  in  the  formation  of  parts  of 
the  larynx   (see  p.  356).     In  the  part  of  the  roof  of  the 

pharynx  Avhich  comes  to  lie  be- 
tween the  openings  of  the  Eusta- 
chian tubes,  a  collection  of  lym- 
phatic tissue  takes  place  beneath 
the  mucous  membrane,  forming 
the  pharyngeal  tonsil,  and  im- 
mediately behind  this  there  is 
formed  in  the  median  line  an  up- 
wardly projecting  pouch,  the  pha- 
ryngeal bursa,  first  certainly  no- 
ticeable  in   embryos   6.5   mm.   in 


Fig.  171. — The  Floor  of 
THE  Pharynx  of  an  Em- 
bryo   OF    2.15    MM. 

/,  Furcula ;  t,  tuberculum 
impar. —  (His.) 


length. 


This  bursa  has  very  generally 
been  regarded  as  the  persistent 
remains  of  Rathke's  pouch  (p.  301),  especially  since  it  is  much 
more  pronounced  in  fetal  than  in  adult  life.  It  has  been 
shown,  however,  that  it  is  formed  quite  independently  of  and 
posterior  to  the  true  Rathke's  pouch  (Killian),  though  what 
its  significance  may  be  is  still  uncertain. 

The  tonsils  are  formed  from  the  epithelium  of  the  lower 
part  of  the  second  branchial  groove.  At  about  the  fourth 
month  solid  buds  begin  to  grow  from  the  epithelium  into 
the  subjacent  mesenchyme,  and  depressions  appear  on  the 
surface  of  this  region.  Later  the  buds  become  hollow  by 
a  cornification  of  their  central  cells,  and  open  upon  the 
floor  of  the  depressions  which  represent  the  crypts  of  the 
tonsil.  In  the  meantime  lymphocytes,  concerning  whose 
origin  there  is  a  difference  of  opinion,  collect  in  the  sub- 
jacent mesenchyme  and  eventually  aggregate  to  form  lym- 


THE    TONSIL.  313 

phatic  follicles  in  close  relation  with  the  buds.  Whether 
the  lymphocytes  wander  out  from  the  blood  into  the  mesen- 
chyme or  are  derived  directly  from  the  epithelium  or  the 
mesenchyme  cells  is  the  cjuestion  at  issue. 

The  tonsil  may  grow  to  a  size  sufficient  to  fill  up  com- 
pletely the  groove  in  which  it  forms,  but  not  infrequently 
a  marked  depression,  the  fossa  supratonsiUaris,  exists  above 
it  and  represents  a  portion  of  the  original  second  branchial 
furrow. 

The  groove  of  Rosemiiiiller,  which  was  at  one  time 
thought  to  be  also  a  remnant  of  the  second  furrow,  is  a 
secondary  depression  which  appears  in  embryos  of  11.5 
cm.  behind  the  opening  of  the  Eustachian  tube,  in  about 
the  region  of  the  third  branchial  furrow. 

The  Development  of  the  Branchial  Epithelial  Bodies. — 
These  are  structures  which  arise  either  as  thickenings  or 
as  outpouchings  of  the  epithelium  lining  the  lower  portions 
of  the  inner  branchial  furrows.  Five  pairs  of  these  struc- 
tures are  developed  and,  in  addition,  there  is  a  single  un- 
paired median  body.  This  last  makes  its  appearance  in 
embryos  of  about  3  mm.,  and  gives  rise  to  the  major  por- 
tion of  the  thyreoid  body.  It  is  situated  immediately  be- 
hind the  anterior  portion  of  the  tongue,  at  the  apex  of  the 
groove  between  this  and  the  posterior  portion,  and  is  first 
a  slight  pouch-like  depression  (Fig.  167).  As  it  deepens, 
its  extremity  becomes  bilobed,  and  after  the  embryo  has 
reached  a  length  of  6  mm.  it  becomes  completely  separated 
from  the  floor  of  the  pharynx.  The  point  of  its  original 
origin  is,  however,  permanently  marked  by  a  circular  de- 
pression, the  foramen  ccucuni  (Fig.  168,  fc).  Later  the 
bilobed  body  migrates  down  the  neck  and  becomes  a  solid 
transversely  elongated  mass  (Fig.  172,  th),  into  the  sub- 
stance of  which  trabeculse  of  connective  tissue  extend,  divid- 
ing it  into  a  network  of  anastomosing  cords  which  later 
28 


314 


THE    BRANCHIAL    EPITHELIAL    BODIES. 


divide  transversely  to  form  follicles.  When  the  embryo 
has  reached  a  length  of  2.6  cm.,  a  cylindrical  outgrowth 
arises  from  the  anterior  surface  of  the  mass,  usually  a  little 
to  the  left  of  the  median  line,  and  extends  up  the  neck  a 
varying  distance,  forming,  when  it  persists  until  adult  life, 
the  so-called  pyramid  of  the  thyreoid  body. 

This  account  of  the  pyramid  follows  the  statements  made  by 
recent  workers  on  the  question  (Tourneux  and  Verdun)  ;  His 


Fig.  172. — Reconstructions  of  the  Branchial  Epithelial  Bodies 
OF  Embryos  of  (A)   14  mm.  and  (B)  26  mm. 

ao,  Aorta;  Ith,  lateral  thyreoid;  ph,  pharynx;  ptlt"  and  pth",  parathy- 
reoids;  th,  thyreoid;  thy,  thymus;  vc,  vena  cava  superior. —  (Tour- 
neux and  Verdun.) 


has  claimed  that  it  is  the  remains  of  the  stalk  connecting  the 
thyreoid  with  the  floor  of  the  pharynx,  and  which  he  terms 
the  thyreo-glossal  duct. 

In  addition  to  this  median  structure,  one  of  the  pairs  of 
the  lateral  evaginations  also  takes  part  in  the  formation  of 
the  thyreoid  body.  These  are  the  lateral  thyreoids  (Fig. 
172,  Ith),  and  they  arise  from  the  posterior  wall  of  the 
fourth  branchial  furrow,  in  embryos  of  about  8  mm.  Sepa- 
rating from  the  furrow,  they  migrate  backward  to  fuse,  in 
embryos  of  about  16  mm.,  with  the  posterior  surface  of  the 
lateral  portions  of  the  median  thyreoid.     They  form,  how- 


THE    BRANCHIAL   EPITHELIAL    BODIES. 


315 


plhm  11 


III  /r 


ever,  only  a  relatively  small  portion  of  the  entire  thyreoid 
(Fig.  173,  thm  IV). 

Two  other  pairs  of  bodies  enter  into  intimate  relations 
with  the  thyreoid,  forming  what  have  been  termed  the 
parathyreoid  bodies 
(Fig.  172,  ptJi}  and 
pth^).  One  of  these 
pairs  arises  as  a  thick- 
ening of  the  anterior 
wall  of  the  fourth  bran- 
chial groove  and  the 
other  comes  from  the 
corresponding  wall  of 
the  third  groove.  The 
members  of  the  former 
pair,  after  separating 
from  their  points  of  ori- 
gin, come  to  lie  on  the 
dorsal  surface  of  the 
lateral  portions  of  the 
thyreoid  body  (Fig.  173, 
pthni  IV)  in  close  prox- 
imity to  the  lateral  thy- 
reoids, while  those  of 
the  other  pair,  passing 
further  backward,  come 
to  rest  behind  the  lower 
border  of  the  thyreoid 
(Fig.  173,  pthm  III). 
The  cells  of  these  bodies 
do  not  become  divided 
into  cords  by  the  ingrowth  of  connective  tissue  to  the  same 
extent  as  those  of  the  thyreoids,  nor  do  they  become  sepa- 
rated into  follicles,  so  that  the  bodies  are  readily  distinguish- 
able by  their  structure  from  the  thyreoid. 


/// 


Fig.     173. — Thyreoid,      Thymus     and 
Epithelial     Bodies     of     a     New- 
born   Child. 
pthm  III  and  pthm  IV,  Parathyreoids ; 
sd,   thyreoid ;    thm  III,  thymus ;    thm 
IV,    lateral    thyreoid. — (Groschuif.) 


3l6  THE    BRANCHIAL    EPITHELIAL    BODIES. 

From  the  posterior  wall  of  the  third  branchial  groove  a 
pair  of  evaginations  develop,  similar  to  those  which  produce 
the  lateral  thyreoids.  These  elongate  greatly,  and  growing 
downward  ventrally  to  the  thyreoid  and  separating  from 
their  points  of  origin,  come  to  lie  below  the  thyreoids,  form- 
ing the  thymus  gland  (Fig.  172,  thy).  As  development 
proceeds  they  pass  further  backward  and  come  eventually 
to  rest  upon  the  anterior  surface  of  the  pericardium.  The 
cavity  which  they  at  first  contain  is  early  obliterated  and  the 
glands  assume  a  lobed  appearance  and  become  traversed  by 
trabeculse  of  connective  tissue.  Lymphocytes,  derived,  ac- 
cording to  some  recent  observations,  directly  from  the  epi- 
thelium of  the  glands,  make  their  appearance  and  gradually 
increase  in  number  until  the  original  epithelial  cells  are  rep- 
resented only  by  a  number  of  peculiar  spherical  structures, 
consisting  of  cells  arranged  in  concentric  layers  and  known 
as  Hassall's  corpuscles. 

The  glands  increase  in  size  until  about  the  fifteenth  year, 
after  which  they  gradually  undergo  degeneration  into  a 
mass  of  fibrous  and  adipose  tissue. 

Finally,  a  pair  of  outgrowths  arise  from  the  floor  of  the 
pharynx  just  behind  the  fifth  branchial  arch,  in  the  region 
where  the  fifth  groove,  if  developed,  would  occur.  These 
post-branchial  bodies,  as  they  have  been  called;  usually 
undergo  degeneration  at  an  early  stage  and  disappear  com- 
pletely, though  occasionally  they  persist  as  cystic  structures 
embedded  in  the  substance  of  the  thyreoid. 

The  relation  of  these  various  structures  to  the  branchial 
grooves  is  shown  by  the  annexed  diagram  (Fig.  174)  ;  and 
from  it,  it  will  be  seen  that  the  bodies  derived  from  the  third 
and  fourth  grooves  are  serially  equivalent.  Comparative  em- 
bryology makes  this  fact  still  more  evident,  since,  in  the  lower 
vertebrates,  each  branchial  groove  contributes  to  the  formation 
of  the  thymus  gland.     The  terminology  used  above   for  the 


THE    BRANCHIAL    EPITHELIAL    BODIES. 


317 


various  bodies  is  that  generally  applied  to  the  mammalian 
organs,  but  it  would  be  better,  for  the  sake  of  comparison  with 
other  vertebrates,  to  adopt  the  nomenclature  proposed  by  Gro- 
schuff,  who  terms  each  lateral  thyreoid  a  thymus  IV,  while 
each  thymus  lobe  is  a  thymus  III.     Similarly  the  parathyreoids 


ihyh 


Fig.  174.— Diagram  showing  the  Origin  of  the  Various  Branchial 
Epithelial  Bodies. 

Ith,  Lateral  thyreoids ;  pp,  postbranchial  bodies ;  pht^  and  phf,  para- 
thyreoids; th,  median  thyreoid;  thy,  thymus;  /  to  IV,  branchial 
grooves. —  {Kohn. ) 


are  termed  parathymus  III  and  IV,  the  term  thyreoid  being 
limited  to  the  median  thyreoid. 

The  Musculature  of  the  Pharynx. — The  pharynx  differs 
from  other  portions  of  the  archenteron  in  the  fact  that  its 
walls  are  furnished  with  voluntary  muscles,  the  principal  of 
which  are  the  constrictors  and  the  stylo-pharyngeus.  This 
peculiarity  arises  from  the  relations  of  the  pharynx  to  the 
branchial  arches.  It  has  been  seen  that  in  the  higher  mam- 
malia the  dorsal  ends  of  the  third,  fourth,  and  fifth  bran- 


3l8  THE    CESOPHAGUS. 

chial  cartilages  disappear;  the  muscles  originally  associated 
with  these  structures  persist,  however,  and  give  rise  to  the 
muscles  of  the  pharynx,  which  consequently  are  innervated 
by  the  ninth  and  tenth  nerves. 

The  Development  of  the  CEsophagus. — From  the  ven- 
tral side  of  the  lower  portion  of  the  pharynx  an  evagination 
develops  at  an  early  stage  which  is  destined  to  give  rise  to 
the  organs  of  respiration;  the  development  of  this  may, 
however,  be  conveniently  postponed  to  a  later  chapter 
(Chapter  XII). 

The  oesophagus  is  at  first  a  very  short  portion  of  the 
archenteron  (Fig.  175,  A),  but  as  the  heart  and  diaphragm 
recede  into  the  thorax,  it  elongates  (Fig.  175,  B)  until  it 
eventually  forms  a  considerable  portion  of  the  digestive 
tract.  Its  endodermal  lining,  like  that  of  the  rest  of  the 
digestive  tract  except  the  pharynx,  is  surrounded  by  splanch- 
nic mesoderm  whose  cells  become  converted  into  non- 
striated  muscular  tissue,  which,  by  the  fourth  month,  has 
separated  into  an  inner  circular  and  an  outer  longitudinal 
layer. 

The  Development  of  the  Stomach  and  Intestines. — 
By  the  time  the  embryo  has  reached  a  length  of  about  3  mm. 
its  constriction  from  the  yolk-sac  has  proceeded  so  far  that 
a  portion  of  the  digestive  tract  anterior  to  the  yolk-sac  can 
be  recognized  as  the  stomach  and  a  portion  posterior  as  the 
intestine.  At  first  the  stomach  is  a  simple,  spindle-shaped 
enlargement  (Fig.  175)  and  the  intestine  a  tube  without 
any  coils  or  bends,  but  since  in  later  stages  the  intestine 
grows  much  more  rapidly  in  length  than  the  abdominal 
cavity,  a  coiling  of  the  intestine  becomes  necessary. 

The  elongation  of  the  stomach  early  produces  changes  in 
its  position,  its  lower  end  bending  over  toward  the  right, 
while  its  upper  end,  owing  to  the  development  of  the  liver, 
is  forced  somewhat  toward  the  left.     At  the  same  time  the 


THE    STOMACH. 


19 


entire  organ  undergoes  a  rotation  about  its  longitudinal  axis 
through  nearly  ninety  degrees,  so  that,  as  the  result  of  the 
combination  of  these  two  changes,  what  was  originally  its 
ventral  border  becomes  its  lesser  curvature  and  what  was 
originally  its  left  surface  becomes  its  ventral  surface. 


Fig.  175. — Reconstructions  of  the  Digestive  Tract  of  Embryos 
of  (a)  4.2  mm.  and  (b)  5  mm. 

all,  Allantois ;  cl,  cloaca;  /,  lung;  It,  liver;  Rp,  Rathke's  pouch;  5", 
stomach;  t.  tongue;  th,  thyreoid  body;  JVd,  Wolffian  duct;  y.  yolk- 
stalk.— (His.) 


Hence  it  is  that  the  left  vagus  nerve  passes  over  the  ventral 
and  the  right  over  the  dorsal  surface  of  the  stomach  in  the 
adult. 

In  the  meantime  the  elongation  of  the  oesophagus  has 
carried  the  stomach  further  away  from  the  lower  end  of  the 


320 


THE    INTESTINE. 


pharynx,  and  from  being  spindle-shaped  it  has  become  more 
pyriform,  as  in  the  adidt. 

The  growth  of  the  intestine  results  in  its  being  thrown 
into  a  loop  opposite  the  point  where  the  yolk-stalk  is  stih 
connected  with  it,  the  loop  projecting  ventrally  into  the  por- 
tion of  the  coelomic  cavity  which  is  contained  within  the 
umbilical  cord,  and  being  placed  so  that  its  upper  limb  lies 
to  the  right  of  the  lower  one.     Upon  the  latter  a  slight 


Fig.   176. — Reconstruction  of  Embryo  of  20  mm. 

C,  Caecum;  K,  kidney;  L,  liver;  S,  stomach;  SC,  suprarenal  bodies; 

W,  mesonephros. —  {Mall.) 

pouch-like  lateral  outgrowth  appears  which  is  the  beginning 
of  the  ccccum  and  marks  the  line  of  union  of  the  future 
small  and  large  intestine.  The  small  intestine,  continuing 
to  lengthen  more  rapidly  than  the  large,  assumes  a  sinuous 
course  (Fig.  176),  in  which  it  is  possible  to  recognize  six 


THE    INTESTINE. 


321 


primary  coils  which  continue  to  be  recognizable  until  ad- 
vanced stages  of  development  and  even  in  the  adult  (Mall). 
The  first  of  these  is  at  first  indistinguishable  from  the 
pyloric  portion  of  the  stomach  and  can  be  recognized  as  the 
duodenum  only  by  the  fact  that  it  has  connected  with  it  the 
ducts  of  the  liver  and  pancreas;  as  development  proceeds, 
however,  its  caliber  diminishes  and  it  assumes  the  appear- 
ance of  a  portion  of  the  intestine. 

The  remaining  coils  elongate  rapidly  and  are  thrown  into 


Fig.  177. — ^Reconstruction  of  the  Intestine  of  an  Embryo  of  19  mm. 
The  Figures  on  the  Intestine  Indicate  the  Primary  Coils. — 
{Mall.) 


♦  numerous  secondary  coils,  all  of  which  are  still  contained 
within  the  coelom  of  the  umbilical  cord  (Fig.  177).  When 
the  embryo  has  reached  a  length  of  about  40  mm.  the  coils 
rather  suddenly  return  to  the  abdominal  cavity,  and  now  the 
caecum  is  thrown  over  toward  the  right,  so  that  it  comes  to 
lie  immediately  beneath  the  liver  on  the  right  side  of  the 
abdominal  cavity,  a  position  which  it  retains  until  about  the 
fourth  month  after  birth  (Treves).  The  portion  of  the 
large  intestine  which  formerly  projected  into  the  umbilical 
coelom  now  lies  transversely  across  the  upper  part  of  the 


322  THE    INTESTINE. 

abdomen,  crossing  in  front  of  the  duodenum  and  having  the 
remaining  portion  of  the  small  intestine  below  it.  The 
elongation  continuing,  the  secondary  coils  of  the  small  in- 
testine become  more  numerous  and  the  lower  portion  of  the 
large  intestine  is  thrown  into  a  loop  which  extends  trans- 
versely across  the  lower  part  of  the  abdominal  cavity  and 
represents  the  sigmoid  flexure  of  the  colon.  At  the  time 
of  birth  this  portion  of  the  large  intestine  is  relatively  much 
longer  than  in  the  adult,  amounting  to  nearly  half  the  entire 
length  of  the  colon  (Treves),  but  after  the  fourth  month 
after  birth  a  readjustment  of  the  relative  lengths  of  the 
parts  of  the  colon  occurs,  the  sigmoid  flexure  becoming 
shorter  and  the  rest  of  the  colon  proportionally  longer, 
whereby  the  caecum  is  pushed  downward  until  it  lies  in  the 
right  iliac  fossa,  the  ascending  colon  being  thus  established. 
When  this  condition  has  been  reached,  the  duodenum, 
after  passing  downward  for  a  short  distance  so  as  to  pass 
dorsally  to  the  transverse  colon,  bends  toward  the  left  and 
the  secondary  coils  derived  from  the  second  and  third  pri- 
mary coils  come  to  occupy  the  left  upper  portion  of  the 
abdominal  cavity.  Those  from  the  fourth  primary  coil  pass 
across  the  middle  line  and  occupy  the  right  upper  part  of 
the  abdomen,  those  from  the  fifth  cross  back  again  to  the 
left  lumbar  and  iliac  regions,  and  those  of  the  sixth  take 
possession  of  the  false  pelvis  and  the  right  iliac  region  (Fig. 
178). 

Slight  variations  from  this  arrangement  are  not  infrequent, 
but  it  occurs  with  sufficient  frequency  to  be  regarded  as  the 
normal.  A  failure  in  the  readjustment  of  the  relative  lengths 
of  the  different  parts  of  the  colon  may  also  occasionally  occur, 
in  which  case  the  caecum  will  retain  its  embryonic  position 
beneath  the  liver. 

The  yolk-stalk  is  continuous  with  the  intestine  at  the 

extremity  of  the  loop  which  extends  out  into  the  umbilical 

coelom,  and  when  the  primary  coils  become  apparent  its 


THE    INTESTINE. 


323 


point  of  attachment  lies  in  the  region  of  the  sixth  coil.  As 
a  rule,  the  caliber  of  the  stalk  does  not  increase  proportion- 
ally with  that  of  the  intestine,  and  eventually  its  embryonic 
portion  disappears  completely.  Occasionally,  however,  this 
portion  of  it  does  partake  of  the  increase  in  size  which 


Fig.   178. — Representation  of  the  Coilings  of  the  Intestine  in  the 
Adult  Condition.    The  Numbers  indicate  the  Primary  Coils. — 


occurs  in  the  intestine,  and  it  forms  a  blind  pouch  of  vary- 
ing length,  known  as  Meckel's  diverticulum  (see  p.  119). 

The  ccEcum  has  been  seen  to  arise  as  a  lateral  outgrowth 
at  a  time  when  the  intestine  is  first  drawn  out  into  the 


324  THE    INTESTINE. 

umbilicus.  During  subsequent  development  it  continues 
to  increase  in  size  until  it  forms  a  conical  pouch  arising 
from  the  colon  just  where  it  is  joined  by  the  small  intes- 
tine (Fig.  179).  The  enlargement  of  its  terminal  portion 
does  not  keep  pace,  however,  with  that  of  the  portion  near- 
est the  intestine,  but  it  becomes  grad- 
ually more  and  more  marked  ofif 
from  it  by  its  lesser  caliber  and  gives 
rise  to  the  vermiform  appendix.  At 
birth  the  original  conical  form  of 
the  entire  outgrowth  is  still  quite  evi- 
dent, though  it  is  more  properly  de- 
scribed as  funnel-shaped,  but  later 
the  proximal  part,  continuing  to  in- 
FiG.    179.— C^cuM    OF     crease  in  diameter  at  the  same  rate 

Embryo  of  10.2  cm.     ^g  ^^iq  colon,  becomes  sharply  sepa- 
c,  Lolon ;  i,  ileum.  .        .  .  , 

rated  from  the  appendix,  formmg  the 
caecum  of  adult  anatomy. 

Up  to  the  time  when  the  embryo  has  reached  a  length  of 
14  mm.,  the  inner  surface  of  the  intestine  is  cjuite  smooth, 
but  when  a  length  of  19  mm.  has  been  reached,  the  mucous 
membrane  of  the  upper  portion  becomes  thrown  into  longi- 
tudinal folds,  and  later  these  make  their  appearance  through- 
out its  entire  length  (Fig.  180).  Later,  in  embryos  of  60 
mm.,  these  folds  break  up  into  numbers  of  conical  processes, 
the  villi,  which  increase  in  number  with  the  development 
of  the  intestine,  the  new  villi  appearing  in  the  intervals 
between  those  already  present. 

A  remarkable  phenomenon  has  recently  been  described  as 
occurring  in  the  duodenum  of  embryos  of  about  12.5  mm.  It 
consists  in  a  rapid  growth  in  the  thickness  of  the  mucous  mem- 
brane, whereby  the  lumen  of  the  intestine  immediately  below 
the  opening  of  the  hepatic  and  pancreatic  ducts  becomes  greatly 
reduced  in  size  and  is  finally  completely  obliterated.  This 
condition  persists  until  the  embryo  has  reached  a  length  of  14.5 


THE  LIVER. 


325 


mm.,  when  the  lumen  again  appears  (Tandler).  This  process 
is  interesting  in  connection  with  the  occasional  occurrence  in 
new-born  children  of  an  atresia  of  the  duodenum. 

The  Development  of  the  Liver. — The  liver  makes  its 
appearance  in  embryos  of  about  3  mm.  as  a  longitudinal 
groove  upon  the  ventral  surface  of  the  archenteron  just 
below  the  stomach  and  between  it  and  the  umbilicus.  The 
endodermal  cells  lining  the  anterior  portion  of  the  groove 
early  undergo  a  rapid  proliferation,  and  form  a  solid  mass 
which  projects  ventrally  into  the  substance  of  a  horizontal 


Fig.  180. — Reconstruction  of  a  Portion  of  the  Intestine  of  an 
Embryo  OF  28  mm.,  showing  the  Longitudinal  Folds  from  which 
THE  Villi  are  Formed. —  (Berry.) 

shelf,  the  septum  transversum  (see  p.  337),  attached  to 
the  ventral  wall  of  the  body.  This  solid  mass  (Fig.  181, 
L)  form  the  beginning  of  the  liver  proper,  while  the  lower 
portion  of  the  groove,  which  remains  hollow,  represents  the 
future  gall-bladder  (Fig.  181,  B).  Constrictions  appear- 
ing between  the  intestine  and  both  the  hepatic  and  cystic 
portions  of  the  organ  gradually  separate  these  from  the 
intestine,  until  they  are  united  to  it  only  by  a  stalk  which 
represents  the  ductus  choledochus  (Fig.  181). 


326 


THE    LIVER. 


The  further  development  of  the  hver,  so  far  as  its  exter- 
nal form  is  concerned,  consists  in  the  rapid  enlargement  of 
the  hepatic  portion  until  it  occupies  the  greater  part  of  the 
upper  half  of  the  abdominal  cavity,  its  ventral  edge  extend- 
ing as  far  down  as  the  umbilicus.  In  the  rabbit  its  sub- 
stance becomes  divided  into  four  lobes  corresponding  to  the 
four  veins,  umbilical  and  omphalo-mesenteric,  which  trav- 
erse it,  and  the  same  condition  occurs  in  the  human  embryo, 
although   the  lobes  are  not  so  clearly  indicated  upon  the 


Fig.    i8i. — Reconstructions   of    the    Liver    Outgrowths   of  Rabbit 

Embryos  of  (A)  5  mm.  and  (5)  of  8  mm. 
B,  Gall-bladder;  d,  duodenum;  DV,  ductus  venosus;  L,  liver;  p,  dorsal 

pancreas;   pm,  ventral   pancreas;   rL,   right  lobe  of  the   liver;   S, 

stomach. —  ( Hammar. ) 

surface  as  in  the  rabbit.  The  two  omphalo-mesenteric  lobes 
are  in  close  apposition  and  may  almost  be  regarded  as  one, 
a  median  ventral  lobe  which  embraces  the  ductus  venosus 
(Fig.  181,  B,  DV),  while  the  umbilical  lobes  are  more  lat- 
eral and  dorsal  and  represent  the  right  {rL)  and  left  lobes 
of  the  adult  liver.  The  remaining  definite  lobes,  the  cau- 
date   (Spigelian)    and    quadrate,    are   of    later    formation, 


THE  LIVER.  327 

standing  in  relation  to  the  vessels  which  cross  the  lower 
surface  of  the  liver. 

The  ductus  choledochus  is  at  first  wide  and  short,  and 
near  its  proximal  end  gives  rise  to  a  small  outgrowth  on 
each  side,  one  of  which  becomes  the  ventral  pancreas  (Fig. 
181,  B,  ptn).  Later  the  duct  elongates  and  becomes  more 
slender,  and  the  gall-bladder  is  constricted  off  from  it,  the 
connecting  stalk   becoming  the   cystic   duct.     The  hepatic 


Fig.    182. — Transverse   Section  through   the  Liver  of  an   Embryo 

OF  Four  Months. 
in^  Intestine;  /,  liver;  W,  Wolffian  body. —  (Toldt  and  Zuckerkandl.) 

ducts  are  apparently  developed  from  the  liver  substance  and 
are  relatively  late  in  appearing. 

Shortly  after  the  hepatic  portion  has  been  differentiated 
its  substance  becomes  permeated  by  numerous  blood-vessels 
(sinusoids)  and  so  divided  into  anastomosing  trabeculae 
(Fig.  182).  These  are  at  first  irregular  in  size  and  shape, 
but  later  they  become  more  slender  and  more  regularly 
cylindrical,  forming  what  have  been  termed  the  hepatic 
cylinders.     In  the  center  of  each  cylinder,  where  the  cells 


328 


THE    LIVER. 


which  form  it  meet  together,  a  fine  canal  appears,  the  begin- 
ning of  a  bile  capillary,  the  cyHnders  thus  becoming  con- 
verted into  tubes  with  fine  kimina.  This  occurs  at  about 
the  fourth  week  of  development  and  at  this  time  a  cross- 
section  of  a  cylinder  shows  it  to  be  composed  of  about  three 
or  four  hepatic  cells  (Fig.  183,  A),  among  which  are  to 
be  seen  groups  of  smaller  cells  (e)  which  are  erythrocytes, 
the  liver  having  assumed  by  this   time  its  hsematopoietic 


Fig.    183. — Transverse   Sections  of  Portions   of  the  Liver  of    (A) 

A  Fetus  of  Six  Months  and  (B)  a  Child  of  Four  Years. 
be,   Bile  capillary;   e,   erythrocyte;    he,   hepatic   cylinder. —  (Toldt    and 

Zuckerkandl.) 


function  (see  p.  238).  This  condition  of  affairs  persists 
until  birth,  but  later  the  cylinders  undergo  an  elongation, 
the  cells  of  which  they  are  composed  slipping  over  one  an- 
other apparently,  so  that  the  cylinders  become  thinner  as 
well  as  longer  and  show  for  the  most  part  only  two  cells 
in  a  transverse  section  (Fig.  183,  B)  ;  and  in  still  later 
periods  the  two  cells,  instead  of  lying  opposite  one  another. 
may  alternate,  so  that  the  cylinders  become  even  more 
slender. 


THE  LIVER. 


329 


,  The  bile  capillaries  seem  to  make  their  appearance  first 
in  cylinders  which  lie  in  close  relation  to  branches  of  the 
portal  vein  (Fig.  184),  and  thence  extend  throughout  the 
neighboring  cylinders,  anastomosing  with  capillaries  devel- 
oping in  relation  to  neighboring  portal  branches.  As  the 
extension  so  proceeds  the  older  capillaries  continue  to  en- 
large and  later  become  transformed  into  bile-ducts  (Fig. 
184,  C),  the  cells  of  the  cylinders  in  which  these  capillaries 
were  situated  becoming  converted  into  the  epithelial  lining 
of  the  ducts. 

The  lobules,  which  form  so  characteristic  a  feature  of 


Fig.  184. — Injected  Bile  Capillaries  of  Pig  Embryos  of  (A)  8  cm., 
(5)   16  CM.,  AND  (C)  OF  Adult  Pig. —  (Hendrickson.) 

the  adult  liver,  are  late  in  appearing,  not  being  fully  devel- 
oped until  some  time  after  birth.  They  depend  upon  the 
relative  arrangement  of  the  branches  of  the  portal  and 
hepatic  veins;  these  at  first  occupy  distinct  territories  01 
the  liver  substance,  being  separated  from  one  another  by 
practically  the  entire  thickness  of  the  liver,  although  of 
course  connected  by  the  capillaries  which  lie  between  the 
hepatic  cylinders.  During  development  the  two  sets  of 
branches  extend  more  deeply  into  the  liver  substance,  each 
invading  the  territory  of  the  other,  but  they  can  readily  be 
29 


330  THE    LIVER. 

distinguished  from  one  another  by  the  fact  that  the  portal 
branches  are  enclosed  within  a  sheath  of  connective  tissue 
(Glisson's  capsule)  which  is  lacking  to  the  hepatic  vessels. 
At  about  the  time  of  birth  the  branches  of  the  hepatic  veins 
give  off  at  intervals  bunches  of  terminal  vessels,  around 
which  branches  of  the  portal  vein  arrange  themselves,  the 
liver  tissue  becoming  divided  up  into  a  number  of  areas 
which  may  be  termed  hepatic  islands,  each  of  which  is  sur- 
rounded by  a  number  of  portal  branches  and  contains 
numerous  dichotomously  branching  hepatic  terminals. 
Later  the  portal  branches  sink  into  the  substance  of  the 
islands,  which  thus  become  lobed,  and  finally  the  sinking  in 
extends  so  far  that  the  original  island  becomes  separated 
into  a  number  of  smaller  areas  or  lobules,  each  containing, 
as  a  rule,  a  single  hepatic  terminal  (the  intralobular  vein) 
and  being  surrounded  by  a  number  of  portal  terminals 
{interlobnlar  veins),  the  two  systems  being  united  by  the 
capillaries  which  separate  the  cylinders  contained  within  the 
area.  The  lobules  are  at  first  very  small,  but  later  they 
increase  in  size  by  the  extension  of  the  hepatic  cylinders. 

Frequently  in  the  human  liver  lobules  are  to  be  found  con- 
taining two  intralobular  veins,  a  condition  which  results  from 
an  imperfect  subdivision  of  a  lobe  of  the  original  hepatic  island. 

The  liver  early  assumes  a  relatively  large  size,  its  weight 
at  one  time  being  equal  to  that  of  the  rest  of  the  body,  and 
though  in  later  embryonic  stages  its  relative  size  dimin- 
ishes, yet  at  birth  it  is  still  a  voluminous  organ,  occupying 
the  greater  portion  of  the  upper  half  of  the  abdominal 
cavity  and  extending  far  over  into  the  left  hypochondrium. 
Just  after  birth  there  is,  however,  a  cessation  of  growth, 
and  the  subsequent  increase  proceeds  at  a  much  slower  rate 
than  that  of  the  rest  of  the  body,  so  that  its  relative  size 
becomes  still  more  diminished  (see  Chap.  XVII).  The 
cessation  of  growth  affects  principally  the  left  lobe  and  is 


THE    PANCREAS; 


■531 


■J 


accompanied  by  an  actual  degeneration  of  portions  of  the 
liver  tissue,  the  cells  disappearing  completely,  while  the 
ducts  and  blood-vessels  originally  present  persist,  the  for- 
mer constituting  the  vasa  aberrantia  of  adult  anatomy. 
These  are  usually  especially  noticeable  at  the  left  edge  of 
the  liver,  between  the 
folds  of  the  left  lateral 
ligament,  but  they  may 
also  be  found  along  the 
line  of  the  vena  cava, 
around  the  gall-bladder, 
and  in  the  region  of  the 
left  longitudinal  fissure. 

The  Development  of 
the  Pancreas. — The  pan- 
creas arises  a  little  later 
than  the  liver,  as  three 
separate  outgrowths,  one 
from  the  dorsal  surface 
of  the  duodenum  (Fig. 
185,  DP)  usually  a  little 
above  the  liver  outgrowth, 
and  one  on  each  side  from 
the  lower  part  of  the  com- 
mon bile-duct.  Of  the 
latter  outgrowths,  that 
upon  the  left  side  (Vps) 
early  begins  to  degenerate 
and  completely  disap- 
pears, while  that  of  the 
right  side  (Vpd)  con- 
tinues its  development  to  form  what  has  been  termed  the 
ventral  pancreas.  Both  this  and  the  dorsal  pancreas  continue 
to  elongate,  the  latter  lying  to  the  left  of  the  portal  vein,  while 


J)J^ 


Fig.  185. — Reconstruction  of  the 
Pancreatic  Outgrowths  of  an 
Embryo  of  7.5  mm. 

D,  Duodenum;  Dc,  ductus  communis 
choledochus  ;  DP,  dorsal  pancreas  ; 
Vpd,  and  Vps,  right  and  left  ven- 
tral pancreas. —  (Helly.) 


332  THE    PANCREAS. 

the  former,  at  first  situated  to  the  right  of  the  vein,  later 
grows  across  its  ventral  surface  so  as  to  come  into  contact 
with  the  dorsal  gland,  with  which  it  fuses  so  intimately 
that  no  separation  line  can  be  distinguished.  The  body 
and  tail  of  the  adult  pancreas  represent  the  original  dorsal 
outgrowth,  while  the  right  ventral  pancreas  becomes  the 
head. 

Both  the  dorsal  and  ventral  outgrowths  early  become 
lobed,  and  the  lobes  becoming  secondarily  lobed  and  this 
lobation  repeating  itself  several  times,  the  compound  tubu- 
lar structure  of  the  adult  gland  is  acquired,  the  very  numer- 
ous terminal  lobules  becoming  the  secreting  acini,  while  the 
remaining  portions  become  the  ducts.  Of  the  principal 
ducts,  there  are  at  first  two;  that  of  the  dorsal  pancreas, 
the  duct  of  Santorini,  opens  into  the  duodenum  on  its  dorsal 
surface,  while  that  of  the  ventral  outgrowth,  the  duct  of 
Wirsung,  opens  into  the  ductus  choledochus.  When  the 
fusion  of  the  two  portions  of  the  gland  occurs,  an  anastomo- 
sis of  branches  of  the  two  ducts  develops  and  the  proximal 
portion  of  the  duct  of  Santorini  usually  degenerates,  so  that 
the  secretion  of  the  entire  gland  empties  into  the  common 
bile-ducts  through  the  duct  of  Wirsung. 

In  the  connective  tissue  which  separates  the  lobules  of 
the  gland,  groups  of  cells  occur,  which  have  no  connection 
with  the  ducts  of  the  gland,  and  form  what  are  termed  the 
areas  of  Langerhans.  They  arise  by  a  differentiation  of 
the  cells  which  form  the  original  pancreatic  outgrowths, 
and  have  been  distinguished  in  the  dorsal  pancreas  of  the 
guinea-pig  while  it  is  still  a  solid  outgrowth.  They  gradu- 
ally separate  from  the  remaining  cells  of  the  outgrowth  and 
come  to  lie  in  the  mesenchyme  of  the  gland  in  groups  into 
which,  finally,  blood-vessels  penetrate. 


LITERATURE.  333 

LITERATURE. 
'^    E.   T.   Bell:     "The  Development  of  the  Thymus,"  Anier.  Journ.   of 

Anat.,  V,   1906. 
^  J.  M.  Berry  :     "  On  the  Development  of  the  Villi  of  the  Human  In- 
testine," Anat.  Anzeiger,  xvi,  1900. 
J.    Bracket:    "  Recherches   sur   le    developpement    du    pancreas   et   du 

foie,"  Journ.  de  I'Anat.  et  de  la  Physiol.,  xxxii,  1896. 
J.   H.   Chievitz  :    "  Beitrage   zur   Entwicklungsgeschichte   der   Speichel- 

driisen,"  Archiv  fur  Anat.  und  Physiol.,  Anat.  Abth.,   1885. 
K.  Groschuff  :     "  Ueber  das  Vorkommen  eines  Thymussegmentes  der 

vierten  Kiementasche  beim  Menschen,"  Anat.  Anzeiger,  xvii,  1900. 
/    J.  A.  Hammar:     "  Einige  Plattenmodelle  zur  Beleuchtung  der  friiheren 

embryonal    Leberentwicklung,"    Arch.    f.    Anat.    und   Phys.,    Anat. 

Abth.,   1893. 
/  J.  A.  Hammar:     "  Studien  iiber  die  Entwicklung  des  Vorderdarms  und 

einiger  angrenzenden   Organe,"  Arch.   f.   mikrosk.  Anat.,  lix  and 

Lx,  1902. 
K.   Helly  :     "  Zur   Entwickelungsgeschichte   der    Pancreasanlagen   und 

Duodenalpapillen   des   Menschen,"  ArcJui'  fiir  mikrosk.  Anat.,  LVi, 

1900. 
/'  K.  Helly:  "Studien  iiber  Langerhanssche  Inseln,"  Arch,  fiir  mikrosk. 

Anat.,  Lxvir,  1907. 
W.    F.    Hendrickson  :     "  The   Development   of  the   Bile-capillaries   as 

revealed  by   Golgi's    Method,"   Johns   Hopkins   Hospital  Bulletin, 

1898. 
'     W.  His  :     "Anatomic  menschlicher  Embryonen,"  Leipzig,  1882-1886. 
V    F.  Keibel:  "Zur  Entwickelungsgeschichte  des  menschlichen  Urogeni- 

tal-apparatus,"  Archiv  fiir  Anat.  und  Physiol.,  Anat.  Abth.,  1896. 
G.    KiLLiAN :     "  Uber   die    Bursa    und   Tonsilla   pharyngea,"    Morphol. 

Jahrbuch,  xiv,  1888. 
'      A.  KoHN :     "  Die  Epithelkorperchen,"  Ergebnisse  der  Anat.  und  Ent- 

wicklungsgesch.,  ix,   1899. 
F.  P.  Mall  :     "  Ueber  die  Entwickelung  des  menschlichen  Darmes  und 

seiner  Lage  beim  Erwachsenen,"  Archiv  fiir  Anat.   und  Physiol., 

Anat.  Abth.,  Supplement,  1897. 
F.  P.  Mall:  "A  Study  of  the  Structural  Unit  of  the  Liver,"  Amer. 

Journ.  of  Anat.,  v,   1906. 
J.  F.  Mece'el:     "  Bildungsgeschichte  des  Darmkanals  der  Saugethiere 

und  namentlich  des  Menschen,"  Archiv  fiir  Anat.  und  Physiol.,  iii, 

1817. 
W.  J.  Otis  :     "  Die   Morphogenese   und  Histogenese  des  Analhockers 

nebst  Bermerkungen  iiber  die  Entwicklung  der  Sphincter  ani  ex- 

ternus  beim  Menschen,  Anat.  Hefte,  xxx,  1906. 
R.   M.   Pearce  :   "  The  Development  of  the  Islands  of  Langerhans  in 

the  Human  Embryo,"  Amer.  Journ.  of  Anat.,  u,  1902. 


334  LITERATURE. 

A.  PoLZL :  "  Zur  Entwicklungsgeschichte  des  menschlichen  Gaumens," 
Anat.  Hefte,  xxvii,   1905. 

C.  Rose:  "  Ueber  die  Entwicklung  der  Ziihne  des  Menschen,"  Archiv 
fiir  mikrosk.  Anat.,  xxxviii,  1891. 

A.  SwAEN :  "  Recherches  sur  le  developement  du  foie,  du  tube  digestif, 
de  I'arriere-cavite  du  peritoine  et  du  mesentere,"  Journ.  de  I'Anat. 
et  de  la  Physiol,  xxxii^  1896,  and  xxxiii,  1897. 

J.  Tandler  :  "  Zur  Entwickelungsgeschichte  des  menschlichen  Duode- 
num in  friihen  Embryonalstadien,"  Morphol.  Jahrbuch,  xxix, 
1900. 

C.  ToLDT  AND  E.  ZucKERKANDL :  "  Ueber  die  Form  und  Texturveran- 
derungen  der  menschlichen  Leber  wahrend  des  Wachsthums,"  Sits- 
uiigsber.  der  kais.  Akad.  VVissensch.  Wien.,  Math.-Naturunss. 
Classe,  Lxxii,  1875. 
^  F.  TouRNEUX  AND  P.  Verdun  :  "  Sur  les  premiers  developpements  de 
la  Thyroide,  du  Thymus  et  des  glandes  parathyroidiennes  chez 
I'homme,"  Journ.  de  I'Anat.  ct  de  la  Physiol,  xxxiii,  1897. 

F.  Treves  :  "  Lectures  on  the  Anatomy  of  the  Intestinal  Canal  and 
Peritoneum  in  Man,"  British  Medical  Journal,  i,   1885. 


CHAPTER   XI 

THE   DEVELOPMENT   OF   THE   PERICARDIUM, 

THE    PLEURO-PERITONEUM    AND    THE 

DIAPHRAGM. 

It  has  been  seen  (p.  241)  that  the  heart  makes  its  appear- 
ance at  a  stage  when  the  greater  portion  of  the  ventral  sur- 
face of  the  intestine  is  still  open  to  the  yolk-sac.  The  ven- 
tral mesoderm  splits  to  form  the  somatic  and  splanchnic 
layers  and  the  heart  develops  as  a  fold  in  the  latter  on  each 
side  of  the  median  line,  projecting  into  the  coelomic  cavity 
enclosed  by  the  two  layers  (Fig.  130,  ^).  As  the  constric- 
tion of  the  anterior  part  of  the  embryo  proceeds  the  two 
heart  folds  are  brought  nearer  together  and  later  meet,  so 
that  the  heart  becomes  a  cylindrical  structure  lying  in  the 
median  line  of  the  body  and  is  suspended  in  the  coelom  by 
a  ventral  band,  the  ventral  mesocardium,  composed  of  two 
layers  of  splanchnic  mesoderm  which  extend  to  it  from  the 
ventral  wall  of  the  body,  and  by  a  similar  band,  the  dorsal 
mesocardium,  which  unites  it  with  the  splanchnic  mesoderm 
surrounding  the  digestive  tract.  The  ventral  mesocardium 
soon  disappears  (Fig.  130,  C)  and  the  dorsal  one  also  van- 
ishes somewhat  later,  so  that  the  heart  comes  to  lie  freely 
in  the  coelomic  cavity,  except  for  the  connections  which  it 
makes  with  the,  body-walls  by  the  vessels  which  enter  and 
arise  from  it. 

The  coelomic  cavity  of  the  embryo  does  not  at  first  com- 
municate with  the  extra-embryonic  coelom,  which  is  formed 
at  a  very  early  period  (see  p.  69),  but  later  when  the  split- 
ting of  the  embryonic  mesoderm  takes  place  the  two  cavities 

335 


zz^ 


THE    PERICARDIUM    AND    PLEURO-PERITONEUM. 


become  continuous  behind  the  heart,  but  not  anteriorly,  since 
the  ventral  wall  of  the  body  is  formed  in  the  heart  region 
before  the  union  can  take  place.     It  is  possible,  therefore, 

to  recognize  two  portions  in  the 
embryonic  coelom,  an  anterior 
one,  the  parietal  cavity  (His), 
which  is  never  connected  later- 
ally with  the  extra-embryonic 
cavity,  and  a  posterior  one,  the 
trunk  cavity,  which  is  so  con- 
nected. The  heart  is  situated  in 
the  parietal  cavity,  a  consider- 
able portion  of  which  is  destined 
to  become  the  pericardial  cavity. 
Since  the  parietal  cavity  lies 
immediately  anterior  to  the  still 
wide  yolk-stalk,  as  may  be  seen 
from  the  position  of  the  heart 
in  the  embryo  shown  in  Fig. 
42,  it  is  bounded  posteriorly  by 
the  yolk-stalk.  This  boundary 
is  complete,  however,  only  in 
the  median  line,  the  cavity  be- 
ing continuous  on  either  side 
of  the  yolk-stalk  with  the  trunk- 
cavity  by  passages  which  have 
been  termed  the  recessus  paric- 


Om 


Rca. 

Fig.  186. — Reconstruction 
OF  A  Rabbit  Embryo  of 
Eight  Days,  with  the 
Pericardial  Cavity  Laid 
Open. 

A,  Auricle;  Aob,  aortic 
bulb;  A.V.,  auriculo-ven- 
tricular  communication ; 
Bp,  ventral  parietal  re- 
cess ;  Om,  omphalo-mes- 
enteric  vein ;  Pc,  peri- 
cardial cavity;  Rca,  dor- 
sal parietal  recess ;  Sv, 
sinus  venosus ;  V,  ventri- 
cle.—(Hw.) 


tales   (Fig.    186,  Bp  and  Rca). 
Passing     forward     toward     the 


heart  in  the  splanchnic  meso- 
derm which  surrounds  the  yolk- 
stalk  are  the  large  omphalo-mesenteric  veins,  one  on  either 
side,  and  these  shortly  become  so  large  as  to  bring  the 
splanchnic  mesoderm  in  which  they  lie  in  contact  with  the 


THE    PERICARDIUM    AND    PLEURO-PERITONEUM.         337 

somatic  mesoderm  which  forms  the  lateral  wall  of  each 
recess.  Fusion  of  the  two  layers  of  mesoderm  along  the 
course  of  the  veins  now  take  place,  and  each  recess  thus 
becomes  divided  into  two  parallel  passages,  which  have  been 
termed  the  dorsal  (Fig.  187,  rpd)  and  ventral  {rpv)  parie- 
tal recesses.  Later  the  two  veins  fuse  in  the  upper  portion 
of  their  course  to  form  the  beginning  of  the  sinus  venosus, 
with  the  result  that  the  ventral  recesses  become  closed  below 
and  their  continuity  with  the  trunk-cavity  is  interrupted,  so 
that  they  form  two  blind  pouches  extending  downward  a 
short  distance  from  the  ventral  portion  of  the  floor  of  the 


VO/n 


vom 


rpv 


Fig, 


187. — Transverse  Sections  of  a  Rabbit  Embryo  showing  the 
Division  of  the  Parietal  Recesses  by  the  Omphalo-mesenteric 
Veins. 

Amnion ;  rp,  parietal  recess ;  rpd  and  rpv,  dorsal  and  ventral  divi- 
sions of  the  parietal  recess ;  vom,  omphalo-mesenteric  vein. — 
{Ravn.^ 


parietal  cavity.     The  dorsal  recesses,  however,  retain  their 
continuity  with  the  trunk-cavity  until  a  much  later  period. 

By  the  fusion  of  the  omphalo-mesenteric  veins  mentioned 
above,  there  is  formed  a  thick  semilunar  fold  which  projects 
horizontally  into  the  coelom  from  the  ventral  wall  of  the 
body  and  forms  the  floor  of  the  ventral  part  of  the  parietal 
recess.  This  is  known  as  the  septum  transversuin,  and 
besides  containing  the  anterior  portions  of  the  omphalo- 
mesenteric veins,  it  also  furnishes  a  passage  by  which  the 
30 


33^        THE    PERICARDIUM    AND    PLEURO-PERITONEUM. 

ductus  Cuvieri,  formed  by  the  union  of  the  jugular  and  car- 
dinal veins,  reach  the  heart.  Its  dorsal  edge  is  continuous 
in  the  median  line  with  the  mesoderm  surrounding  the  diges- 
tive tract  just  opposite  the  region  where  the  liver  outgrowth 
will  form,  but  laterally  this  edge  is  free  and  forms  the  ven- 
tral walls  of  the  dorsal  parietal  recess.  An  idea  of  the  rela- 
tions of  the  septum  at  this  stage  may  be  obtained  from  Fig. 


ain 


Fig.    i88. — Reconstruction    from    a   Rabbit   Embryo   of   Nine   Days 

SHOWING    THE    SePTUM    TrANSVERSUM     FROM    AbOVE. 

am,  Amnion ;  at,  atrium ;  dc,  ductus  Cuvieri ;  rpd,  dorsal  parietal  recess. 

—  (Ravn.) 

i8S,  which  represents  the  anterior  surface  of  the  septum, 
together  with  the  related  parts,  in  a  rabbit  embryo  of  nine 
days. 

The  Separation  of  the  Pericardial  Cavity. — The  septum 
transversum  is  at  first  almost  horizontal,  but  later  it  becomes 


THE    DIAPHRAGM.  339 

decidedly  oblique  in  position,  a  change  associated  with  the 
backward  movement  of  the  heart.  As  the  closure  of  the 
ventral  wall  of  the  body  extends  posteriorly  the  ventral  edge 
of  the  septum  gradually  slips  downward  upon  it,  while  the 
dorsal  edge  is  held  in  its  former  position  by  its  attachment 
to  the  wall  of  the  digestive  tract  and  the  ductus  Cuvieri. 
The  anterior  surface  of  the  septum  thus  comes  to  look  ven- 
trally  as  well  as  forward,  and  the  parietal  cavity,  having 
taken  up  into  itself  the  blind  pouches  which  represented  the 
ventral  recesses,  comes  to  lie  to  a  large  extent  ventral  to 
the  posterior  recesses.  As  may  be  seen  from  Fig.  i88,  the 
ductus  Cuvieri,  as  they  bend  from  the  lateral  walls  of  the 
body  into  the  free  edges  of  the  septum,  form  a  marked 
projection  which  diminishes  considerably  the  opening  of  the 
dorsal  recesses  into  the  parietal  cavity.  In  later  stages  this 
projection  increases  and  from  its  dorsal  edge  a  fold,  which 
may  be  regarded  as  a  continuation  of  the  free  edge  of  the 
septum,  projects  into  the  upper  portions  of  the  recesses  and 
eventually  fuses  with  the  median  portion  of  the  septum 
attached  to  the  wall  of  the  gut.  In  this  way  the  parietal 
cavity  becomes  a  completely  closed  sac,  and  is  henceforward 
known  as  the  pericardial  caz'ify,  the  original  coelom  being 
now  divided  into  two  portions,  (i)  the  pericardial,  and  (2) 
the  pleiiro-peritoneal  cavities,  the  latter  consisting  of  the 
abdominal  coelom  together  with  the  two  dorsal  parietal 
recesses  which  have  been  separated  from  the  pericardial 
(parietal)  cavity  and  are  destined  to  be  converted  into  the 
pleural  cavities. 

The  Formation  of  the  Diaphragm. — It  is  to  be  remem- 
bered that  the  attachment  of  the  transverse  septum  to  the 
ventral  wall  of  the  digestive  tract  is  opposite  the  point  where 
the  liver  outgrowth  develops.  When,  therefore,  the  out- 
growth appears,  it  pushes  its  way  into  the  substance  of  the 
septum,  which  thus  acquires  a  ver}-  considerable  thickness, 


340 


THE    DIAPHRAGM. 


especially  toward  its  dorsal  edge,  and  it  furthermore  be- 
comes differentiated  into  two  layers,  an  upper  one,  which 
forms  the  floor  of  the  ventral  portion  of  the  pericardial 
cavity  and  encloses  the  Cuvierian  ducts,  and  a  lower  one 
which  contains  the  liver.  The  upper  layer  is  comparatively 
thin,  while  the  lower  forms  the  greater  part  of  the  thickness 
of  the  septum,  its  posterior  surface  meeting  the  ventral  wall 
of  the  abdomen  at  the  level  of  the  anterior  margin  of  the 
umbilicus  (Fig.  189,  A). 

In  later  stages  of  development  the  layer  containing  the 


Fig.  189. — Diagrams  of  (A)  a  Sagittal  Section  of  an  Embryo  show- 
ing THE  Liver  Enclosed  within  the  Septum  Transversum  ;  (B) 
a  Frontal  Section  of  the  Same;  (C)  a  Frontal  Section  of  a 
Later  Stage  when  the  Liver  has  Separated  from  the  Dia- 
phragm. 

All,  Allantois;  CI,  cloaca;  D,  diaphragm;  Li,  liver;  Ls,  falciform  liga- 
ment of  the  liver ;  M,  mesentery ;  Mg,  mesogastrium ;  Pc,  peri- 
cardium ;  S,  stomach ;  ST,  septum  transversum ;   U,  umbilicus. 


liver  becomes  separated  from  the  upper  layer  by  two  grooves 
which,  appearing  at  the  sides  and  ventrally  immediately 
above  the  liver  (Fig.  189,  B),  gradually  deepen  toward  the 
median  line  and  dorsally.  These  grooves  do  not,  however, 
quite  reach  the  median  line,  a  portion  of  the  lower  layer 
of  the  septum  being  left  in  this  region  as  a  fold,  situated 
in  the  sagittal  plane  of  the  body  and  attached  above  to  the 


THE    DIAPHRAGM.  341 

posterior  surface  of  the  upper  layer  and  below  to  the  ante- 
rior surface  of  the  liver,  beyond  which  it  is  continued  down 
the  ventral  wall  of  the  abdomen  to  the  umbilicus  (Fig.  189, 
C,  Ls).  This  is  the  falciform  ligament  of  the  liver  of  adult 
anatomy,  and  in  the  free  edge  of  its  prolongation  down  the 
ventral  wall  of  the  abdomen  the  umbilical  vein  passes  to  the 
under  surface  of  the  liver,  while  the  free  edge  of  that  por- 
tion which  lies  between  the  liver  and  the  digestive  tract  con- 
tains the  omphalo-mesenteric  (portal)  vein,  the  common 
bile-duct,  and  the  hepatic  artery.  The  diagram  given  in 
Fig.  184  will,  it  is  hoped,  make  clear  the  mode  of  formation 
and  the  relation  of  this  fold,  which,  in  its  entirety,  consti- 
tutes what  is  sometimes  termed  the  ventral  mesentery. 

And  not  only  do  the  grooves  fail  to  unite  in  the  median 
line,  but  they  also  fail  to  completely  separate  the  liver  from 
the  upper  layer  of  the  septum  dorsally,  the  portion  of  the 
lower  layer  which  persists  in  this  region  forming  the  coro- 
nary ligament  of  the  liver.  The  portion  of  the  lower  laver 
which  forms  the  roof  of  the  grooves  becomes  the  layer  of 
peritoneum  covering  the  posterior  surface  of  the  upper  layer 
(which  represents  the  diaphragm),  while  the  portion  which 
remains  connected  with  the  liver  constitutes  its  peritoneal 
investment. 

In  the  meantime  changes  have  been  taking  place  in  the 
upper  layer.  As  the  rotation  of  the  heart  occurs,  so  that 
its  atrial  portion  comes  to  lie  anterior  to  the  ventricle, 
the  Cuvierian  ducts  are  drawn  away  from  the  septum  and 
penetrate  the  posterior  wall  of  the  pericardium,  the  sepa- 
ration being  assisted  by  the  continued  descent  of  the  at- 
tachment of  the  edge  of  the  septum  to  the  ventral  wall  of 
the  body.  During  the  descent,  when  the  upper  layer  of 
the  septum  has  reached  the  level  of  the  fourth  cervical  seg- 
ment, portions  of  the  myotomes  of  that  segment  become 
prolonged  into  it  and  the  layer  assumes  the  characteristics 


342  THE    PLEUR.E. 

of  the  diaphragm,  the  supply  of  whose  musculature  from 
the  fourth  cervical  nerves  is  thus  explained. 

The  Plew'cc. — The  diaphragm  is  as  yet,  however,  incom- 
plete dorsally,  where  the  dorsal  parietal  recesses  are  still  in 
continuity  with  the  trunk-cavity.  With  the  increase  in 
thickness  of  the  septum  transversum,  these  recesses  have 
acquired  a  considerable  length  antero-posteriorly,  and  into 
their  upper  portions  the  outgrowths  from  the  lower  part 
of  the  pharynx  which  form  the  lungs  (see  page  353) 
begin  to  project.  The  recesses  thus  become  transformed 
into  the  pleural  cavities,  and  as  the  diaphragm  continues 
to  descend,  slipping  down  the  ventral  wall  of  the  body  and 
drawing  with  it  the  pericardial  cavity,  the  latter  comes  to 
lie  entirely  ventral  to  the  pleural  cavities.  The  free  borders 
of  the  diaphragm,  which  now  form  the  ventral  boundaries 
of  the  openings  by  which  the  pleural  and  peritoneal  cavities 
communicate,  begin  to  approach  the  dorsal  wall  of  the  body, 
with  which  they  finally  unite  and  so  complete  the  separation 
of  the  cavities.  The  pleural  cavities  continue  to  enlarge 
after  their  separation  and,  extending  laterally,  pass  between 
the  pericardium  and  the  lateral  walls  of  the  body  until  they 
finally  almost  completely  surround  the  pericardium.  The 
intervals  between  the  two  pleurae  form  what  are  termed  the 
inediastina. 

The  downward  mo\Tment  of  the  septum  trans\'ersum 
extends  through  a  very  considerable  interval,  which  may 
be  appreciated  from  the  diagram  shown  in  Fig.  190.  From 
this  it  may  be  seen  that  in  early  embryos  the  septum  is  situ- 
ated just  in  front  of  the  first  cervical  segment  and  that  it 
lies  very  obliquely,  its  free  edge  being  decidedly  posterior 
to  its  ventral  attachment.  When  the  downward  displace- 
ment occurs,  the  ventral  edge  at  first  moves  more  rapidly 
than  the  dorsal,  and  soon  comes  to  lie  at  a  much  lower  level. 
The  backward  nio\'ement  continues  throughout  the  entire 


THE    PERITONEUM. 


343 


1  (iuov%ia£ 


length  of  the  cervical  and  thoracic  regions,  and  when  the 
level  of  the  tenth  thoracic  segment  is  reached  the  separation 
of  the  pleural  and  peritoneal  cavities  is  completed,  and  then 
the  dorsal  edge  begins  to  descend  more  rapidly  than  the 
ventral,  so  that  the  diaphragm  again  becomes  oblique  in  the 
same  sense  as  in  the  beginning, 
a  position  which  it  retains  in 
the  adult. 

The  Development  of  the 
Peritoneum.  —  The  peritoneal 
cavity  is  developed  from  the 
trunk-cavity  of  early  stages 
and  is  at  first  in  free  commu- 
nication on  all  sides  of  the 
yolk-stalk  with  the  extra-em- 
bryonic coelom.  As  the  ven- 
tral wall  of  the  body  develops 
the  two  cavities  become  more 
and  more  separated,  and  with 
the  formation  of  the  umbili- 
cal cord  the  separation  is  com- 
plete. Along  the  mid-dorsal 
line  of  the  body  the  archenteron 
forms  a  projection  into  the 
cavity  and  later  moves  further 
out  from  the  body-wall  into 
the  cavity,  pushing  in  front  of  it  the  peritoneum,  which 
thus  comes  to  surround  the  intestine,  forming  its  serous 
coat,  and  from  it  is  continued  back  to  the  dorsal  body-wall 
forming  the  mesentery. 

It  has  already  been  seen  that  on  the  separation  of  the 
liver  from  the  septum  transversum,  the  tissue  of  the  latter 
gives  rise  to  the  peritoneal  covering  of  the  liver  and  of  the 
posterior  surface  of  the  diaphragm,  and  also  to  the  ventral 


Fig.  190. — Diagram  showing 
THE  Position  of  the  Dia- 
phragm IN  Embryos  of  Dif- 
ferent Ages. —  {Mall.) 


344  THE    PERITONEUM. 

mesentery.  When  the  separation  is  taking  place,  the  rota- 
tion of  the  stomach  already  described  (p.  319)  occurs, 
with  the  result  that  the  portion  of  the  ventral  mesentery 
which  stretches  between  the  lesser  curvature  of  the  stomach 
and  the  liver  shares  in  the  rotation  and  comes  to  lie  in  a 
plane  practically  at  right  angles  with  that  of  the  suspensory 
ligament,  its  surfaces  looking  dorsally  and  ventrally  and  its 
free  edge  being  directed  toward  the  right.  This  portion 
of  the  ventral  mesentery  forms  what  is  termed  the  lesser 
omentum,  and  between  it  and  the  dorsal  surface  of  the 
stomach  as  the  ventral  boundaries,  and  the  dorsal  wall  of 
the  abdominal  cavity  dorsally,  there  is  a  cavity,  whose  floor 
is  formed  by  the  dorsal  mesentery  of  the  stomach,  the  meso- 
gastrium,  the  roof  by  the  under  surface  of  the  left  half  of 
the  liver,  while  to  the  right  it  communicates  with  the  gen- 
eral peritoneal  cavity  dorsal  to  the  free  edge  of  the  lesser 
omentum.  This  cavity  is  known  as  the  bursa  omcntalis 
(lesser  sac  of  the  peritoneum),  and  the  opening  into  it  from 
the  general  cavity  or  greater  sac  is  termed  the  epiploic  fora- 
men (foramen  of  Winslow).  Later,  the  floor  of  the  lesser 
sac  is  drawn  downward  to  form  a  broad  sheet  of  peritoneum 
lying  ventral  to  the  coils  of  the  small  intestine  and  consist- 
ing of  four  layers;  this  represents  the  great  omentum  of 
adult  anatomy  (Fig.  194). 

Although  the  form  assumed  by  the  bursa  omentalis  is 
associated  with  the  rotation  of  the  stomach,  it  seems  prob- 
able that  its  real  origin  is  independent  of  that  process  (Bro- 
man).  The  subserous  tissue  of  the  transverse  septum  is  at 
first  thick  and  includes  not  only  the  liver,  but  also  the  pan- 
creas and  the  portion  of  the  digestive  tract  which  becomes 
the  stomach  and  the  upper  part  of  the  duodenum  (Fig. 
189,  A).  The  shrinkage  of  this  tissue  by  which  these 
organs  become  separated  from  the  septum  cannot  take  place 
evenly  on  account  of  the  relations  which  the  organs  bear 


THE    PERITONEUM, 


345 


to  one  another,  so  that  on  the  right  side  certain  peritoneal 
recesses  are  formed,  one  between  the  right  lung  and  the 
stomach,  a  second  between  the  liver  and  the  stomach,  and 
a  third  between  the  pancreas  and  the  same  structure.  In 
man  these  three  recesses  communicate  with  one  another  to 
form  the  primary  bursa  omentalis,  and  open  by  a  common 
epiploic  foramen  into  the  general  peritoneal  cavity.  The 
rotation  of  the  stomach,  which  takes  place  later,  merely 
serves  to  modify  and  intensify  the  original  bursa. 

In  the  human  embryo  a  small  recess  also  forms  upon  the 
left  side  between  the  left  lung  and  the  stomach,  but  it  later 
becomes  obliterated. 

Below  the  level  of  the  upper  part  of  the  duodenum  the 
ventral  mesentery  is  wanting; 
only  the  dorsal  mesentery  occurs. 
So  long  as  the  intestine  is  a 
straight  tube  the  length  of  the 
intestinal  edge  of  this  mesentery 
is  practically  equal  to  that  of  its 
dorsal  attached  edge.  The  intes- 
tine, however,  increasing  in  length 
much  more  rapidly  than  the  ab- 
dominal walls,  the  intestinal  edge 
of  the  mesentery  soon  becomes 
very  much  longer  than  the  attached 

edge,  and  when  the  intestine  grows  Fig.  191.— Diagram  show 
out  into  the  umbilical  coelom  the 
mesentery  accompanies  it  (Fig. 
191).  As  the  coils  of  the  intestine 
develop,  the  intestinal  edge  of  the 
mesentery  is  thrown  into  corres- 
ponding folds,  and  on  the  return 

of  the  intestine  to  the  abdominal  cavity  the  mesentery  is 
thrown  into  a  somewhat  funnel-like  form  by  the  twisting  of 


iNG  THE  Arrangement 
OF  THE  Mesentery  and 
Visceral  Branches  of 
THE  Abdominal  Aorta 
in  an  Embryo  of  Six 
Weeks. 
p,  Pancreas ;  S,  stomach ; 
Sp,  spleen. —  (Toldt.) 


346 


THE    PERITONEUM. 


the  intestine  to  form  its  primary  loop  (Fig.  192).  All  that 
portion  of  the  mesentery  which  is  attached  to  the  part  of 
the  intestine  which  will  later  become  the  jejunum,  ileum, 
ascending  and  transverse  colon,  is  attached  to  the  body- 
wall  at  the  apex  of  the  funnel,  at  a  point  which  lies  to  the 
left  of  the  duodenum. 

Up  to  this  stage  or  to  about  the  middle  of  the  fourth 
month  the  mesentery  has  retained  its  attachment  to  the 
median  line  of  the  dorsal  wall  of  the  abdomen  throughout 

A  3 


■  md' 


Fig.  192. — Diagrams  Illustrating  the  Development  of  the  Great 
Omentum  and  the  Transverse  Mesocolon. 

Ud,  Caecum;  dd,  small  intestine;  dg,  yolk-stalk;  di.  colon;  du,  duode- 
num; gc,  greater  curvature  of  stomach;  gg,  bile  duct;  gn,  meso- 
gastrium ;  k,  point  where  the  loops  of  the  intestine  cross ;  mc, 
mesocolon;  md,  rectum;  mes,  mesentery;  wf,  vermiform  appendix. 
—  {H  cj-tzvig.) 

its  entire  length,  but  later  fusions  of  certain  portions 
occur,  whereby  the  original  condition  is  greatly  modified. 
One  of  the  earliest  of  these  fusions  takes  place  at  the  apex 
of  the  funnel,  where  the  portion  of  the  mesentery  which 
passes  to  the  transverse  colon  and  arches  over  the  duode- 
num fuses  with  the  ventral  surface  of  the  latter  portion  of 
the   intestine  and   also  with   the  peritoneum   covering  the 


THE    PERITONEUM. 


347 


dorsal  wall  of  the  abdomen  both  to  the  right  and  to  the 
left  of  the  duodenum.  In  this  way  the  attachment  of  the 
traiisz'crse  incsocolon  takes  the  form  of  a  transverse  line 
instead  of  a  point,  and  this  portion  of  the  mesentery  divides 
the  abdominal  cavity  into  two  portions,  the  upper  (ante- 
rior) of  which  contains  the  liver  and  stomach,  while  the 
lower  contains  the  remainder  of  the  digestive  tract  with 
the  exception  of  the  duodenum.  By  passing  across  the 
ventral  surface  of  the  duodenum  and  fusing  with  it,  the 


Fig.  193. — Diagrams  Illustrating  the  Manner  in  which  the  Fixa- 
tion OF  the  Descending  Colon  (C)  takes  Place. 

transverse  mesocolon  forces  that  portion  of  the  intestine 
against  the  dorsal  wall  of  the  abdomen  and  fixes  it  in  that 
position,  and  its  mesentery  thereupon  degenerates,  becom- 
ing subserous  areolar  tissue,  the  duodenum  assuming  the 
retroperitoneal  position  which  characterizes  it  in  the  adult. 
The  descending  colon,  which  on  account  of  the  width 
of  its  mesentery  is  at  first  freely  movable,  lies  well  over  to 
the  left  side  of  the  abdominal  cavity,  and  in  consequence 
the  left  layer  of  its  mesentery  lies  in  contact  with  the 
parietal  layer  of  the  peritoneum.  A  fusion  of  these  two 
layers,  beginning  near  the  middle  line  and  thence  extend- 


348  THE    PERITONEUM. 

ing  outward,  takes  place,  the  fused  layers  becoming  con- 
verted into  connective  tissue,  and  this  portion  of  the  colon 
thus  loses  its  mesentery  and  becomes  fixed  to  the  abdominal 
wall.  The  process  by  which  the  fixation  is  accomplished 
may  be  understood  from  the  diagrams  which  constitute 
Fig.  193.  When  the  ascending  colon  is  formed,  its  mesen- 
tery undergoes  a  similar  fusion,  and  it  also  becomes  fixed 
to  the  abdominal  wall. 

The  fusion  of  the  mesentery  of  the  ascending  and  descend- 
ing colon  remains  incomplete  in  a  considerable  number  of  cases 
(one  fourth  to  one  third  of  all  cases  examined),  and  in  these 
the  colons  are  not  perfectly  fixed  to  the  abdominal  wall.  It 
may  also  be  pointed  out  that  the  caecum  and  appendix,  being 
primarily  a  lateral  outpouching  of  the  intestine,  do  not  possess 
any  true  mesentery,  but  are  completely  enclosed  by  peritoneum. 
Usually  a  falciform  fold  of  peritoneum  may  be  found  extend- 
ing along  one  surface  of  the  appendix  to  become  continuous 
with  the  left  layer  of  the  mesentery  of  the  ileum.  This,  how- 
ever, is  not  a  true  mesentery,  and  is  better  spoken  of  as  a 
mesenteriole. 

One  other  fusion  is  still  necessary  before  the  adult  condi- 
tion of  the  mesentery  is  acquired.  The  great  omentum 
consists  of  two  folds  of  peritoneum  which  start  from  the 
greater  curvature  of  the  stomach  and  pass  downward  to 
be  reflected  up  again  to  the  dorsal  wall  of  the  abdomen, 
-which  they  reach  just  anterior  to  (above)  the  line  of  attach- 
ment of  the  transverse  mesocolon  (Fig.  194,  A).  At  first 
the  attachment  of  the  omentum  is  vertical,  since  it  repre- 
sents the  mesogastrium,  but  later,  by  fusion  with  the  parietal 
peritoneum,  it  assumes  a  transverse  direction,  while  at  the 
same  time  the  pancreas,  which  originally  lay  between  the 
two  folds  of  the  mesogastrium,  is  carried  dorsally  and 
comes  to  have  a  retroperitoneal  position  in  the  line  of  at- 
tachment of  the  omentum.  By  this  change  the  lower  layer 
of  the  omentum  is  brought  in  contact  with  the  upper  layer 
of  the  transverse  mesocolon  and  a  fusion  and  degeneration 


THE    PERITONEUM. 


349 


of  the  two  results  (Fig.  194,  B),  a  condition  which  brings 
it  about  that  the  omentum  seems  to  be  attached  to  the  trans- 
verse colon  and  that  the  pancreas  seems  to  lie  in  the  line  of 
attachment  of  the  transverse  mesocolon.  This  mesentery, 
as  it  occurs  in  the  adult,  really  consists  partly  of  a  portion 
of  the  original  transverse  mesocolon  and  partly  of  a  layer 
of  the  great  omentum. 

By  these  various  changes  the  line  of  attachment  of  the 


Fig.  194. — Diagrams  showing  the  Development  of  the  Great  Omen- 
tum AND  ITS  Fusion  with  the  Transverse  Mesocolon. 
B,  Bladder;  c,  tranverse  colon;  d,  duodenum;  Li,  liver;  p,  pancreas; 
R,   rectum;   S,  stomach;    U,   uterus. —  (After  Allen   Thomson.) 


mesentery  to  the  dorsal  wall  of  the  body  has  become  some- 
what complicated  and  has  departed  to  a  very  considerable 
extent  from  its  original  simple  vertical  arrangement.  If 
all  the  viscera  be  removed  from  the  body  of  an  adult  and 


350  THE    PERITONEUM. 

the  mesentery  be  cut  close  to  the  Hue  of  its  attachment, 
the  course  of  the  latter  will  be  seen  to  be  as  follows :  De- 
scending- from  the  under  surface  of  the  diaphragm  are  the 
lines  of  attachment  of  the  suspensory  ligament,  which  on 
reaching-  the  liver  spread  out  to  become  the  coronary  and 
lateral  ligaments  of  that  organ.  At  about  the  mid-dorsal 
line  these  lines  become  continuous  with  those  of  the  meso- 
gastrium  which  curve  downward  toward  the  left  and  are 
continued  into  the  transverse  lines  of  the  transverse  meso- 
colon. Between  these  last,  in  a  slight  prolongation,  there 
may  be  seen  to  the  right  the  cut  end  of  the  first  portion 
of  the  duodenum  as  it  passes  back  to  the  dorsal  wall  of  the 
abdomen,  and  at  about  the  mid-dorsal  line  the  cut  ends  of 
its  last  part  become  visible  as  it  passes  ventrally  again  to 
become  the  jejunum.  From  the  transverse  mesocolon 
three  lines  of  attachment  pass  downward ;  the  two  lateral 
broad  ones  represent  the  lines  of  fixation  of  the  ascending 
and  descending  colons,  while  the  narrower  median  one, 
which  curves  to  the  right,  represents  the  attachment  of  the 
mesentery  of  the  small  intestine  other  than  the  duodenum. 
Finally,  from  the  lower  end  of  the  fixation  line  of  the 
descending  colon  the  mesentery  of  the  sigmoid  is  continued 
downward. 

The  special  developments  of  the  peritoneum  in  connec- 
tion with  the  genito-urinary  apparatus  will  be  considered 
in  Chapter  XIII. 

LITERATURE. 

I.  Broman  :  "  Ueber  die  Entwicklung  und  Bedeutung  der  Mesenterien 

und  der  Korperhohlen  bei   den   Wirbeltieren,"  Ergebn.   dcr  Anat. 

u.  Entiv.,  XV,  igo6. 
A.  Bracket:  "Die  Entwickelung  der  grossen  Korperhohlen  und  ihre 

Trennung    von    Einander,"    Ergebnisse    der    Anat.    und    Entzvick- 

elungsgesch.,  vu,    1898. 
W.   His:    "  Mittheilungen   zur   Embryologie   der    Siiugethiere   und   des 

Menschen,"   ArcJiiv  fiir  Anat.    mid  Physiol.,   Anat.   Abth.,    1881. 
F.  P.  Mall:  "Development  of  the  Human  Coelom,"  Jounml  of  Mor- 

phoL,  XII,   1897. 


LITERATURE.  35 1 


/ 


E.  Ravn  :  "  Ueber  die  Bildung  der  Scheidewand  zwischen  Brust-  und 

Bauchhohle     in     Saugethierembryonen,"     Archiv    fiir    Anat.     und 
Physiol,  Anat.  Abth.,   1889. 

A.  SwAEN :  "  Recherclies  sur  le  developpement  du  foie,  du  tube  di- 
gestif, de  rarriere-cavite  du  peritoine  et  du  mesentere,"  Joiirn.  de 
I' Anat.  et  de  la  Physiol.,  xxxiii,  1896;  xxxin,  1897. 

C.  ToLDT :  "  Bau  und  Wachstumsveranderungen  der  Gekrose  des 
menschlichen  Darmkanals,"  Denkschr.  der  kais.  Akad.  Wissensch. 
Wien,  Math.-Nafuriviss.  Classe,  XLi,  1879. 
y^  C.  ToLDT :  "  Die  Darmgekrose  und  Netze  im  gesetzmassigen  und  gesetz- 
widrigen  Zustand,"  Denkschr.  der  kais.  Akad.  Wissensch.  Wien, 
M atli.-N aturzmss.  Classe,  lvi,  1889. 

F.  Treves  :   "  Lectures   on  the  Anatomy  of  the   Intestinal   Canal   and 

Peritoneum,"  British  Medical  Journal,  i,  1885. 


CHAPTER    XII. 


THE    DEVELOPMENT   OF   THE    ORGANS    OF 
RESPIRATION. 

The  Development  of  the  Lungs. — The  first  indication 
of  the  hnigs  and  trachea  is  found  in  embryos  of  about  32 
mm.  in  the  form  of  a  groove  on  the  ventral  surface  of  the 
oesophagus,  at  first  extending  almost  the  entire  length  of 
that  portion  of  the  digestive  tract.  As  the  oesophagus 
lengthens  the  lung  groove  remains  connected  w^ith  its  upper 

portion  (Fig.  175,  A), 
and  furrows  which  appear 
along  the  line  of  junction 
of  the  groove  and  the  oeso- 
phagus gradually  deepen 
and  separate  the  two  struc- 
tures (Fig.  175,  B).  The 
separation  takes  place  earli- 
est at  the  lower  end  of  the 
groove  and  thence  extends 
upward,  so  that  the  groove 
is  transformed  into  a  cylin- 
drical pouch  lying  ventral 
i4°ro,  ^.%lSs;  «"?: p"4,"i   to  the  oesophagus  and  dor- 

recess;    VOm,   omphalo-mesenleric    gal  to   the  heart  and  open- 
vein. —  (Toldt.)  .  -,1       ,_i  1 

mg    With    the    oesopliagus 

into  the  terminal  portion  of  the  pharynx. 

Soon  after  the  separation  of  the  groove  from  the  oesoph- 
agus its  lower  end  Ijecomes  enlarged  and  biloljed,  and  since 
this   lower  end   lies,   with   the  oesophagus,   in   the  median 

352 


Fig.     195. — Portion    of    a     Section 

THROUGH       AN       EmBRYO       OF      THE 

Fourth  Week. 
A,   Aorta;    DC,   ductus    Cuvieri ;    L, 


THE   LUNGS. 


353 


attached  portion  of  the  dorsal  edge  of  the  septum  transver- 
sum,  the  lobes,  as  they  enlarge,  project  into  the  dorsal 
parietal  recesses  (Fig.  195),  and  so  become  enclosed  within 
the  peritoneal  lining  of  the  recesses  which  later  become  the 
pleural  cavities. 

The  lobes,  which  represent  the  lungs,  do  not  long  remain 
simple,  but  bud-like  processes  arise  from  their  cavities,  three 
appearing  in  the  right  lobe  and  two  in  the  left  (Fig.   196, 


---// 


J'/ 


Fig.    196. — Reconstruction   of   the   Lung    Outgrowths    of    Embryos 

OF     {A)     10,     (5)     8.5,    AND     (C)     10.5    MM. 

Ap,  Pulmonary  artery;  Ep,  eparterial  bronchus;   Vp,  pulmonary  vein; 
/,   second   lateral   bronchus;    II,  main   bronchi. —  (His.) 


A),  and  as  these  increase  in  size  and  give  rise  to  additional 
outgrowths,  the  structure  of  the  lobes  rapidly  becomes  com- 
plicated (Fig.  196,  B  and  C). 

The  lower  primary  process  on  each  side  may  be  regarded 
as  a  prolongation  of  the  bronchus,  while  the  remaining 
process  or  processes  represent  lateral  outgrowths  from  it. 
Considerable  difference  of  opinion  has  existed  as  to  the 
nature  of  the  further  branching  of  the  bronchi,  some  authors 
regarding  it  as  a  succession  of  dichotomies,  one  branch  of 
each  of  these  placing  itself  so  as  to  be  in  the  line  of  the 
31 


354  THE    LUNGS. 

original  main  bronchus,  while  the  other  comes  to  resemble 
a  lateral  outgrowth,  and  other  observers  have  held  that 
the  main  bronchus  has  an  uninterrupted  growth,  all  other 
branches  being  lateral  outgrowths  from  it,  and  the  branch- 
ing therefore  a  monopodial  process.  The  recent  thorough 
study  by  Flint  of  the  development  of  the  lung  of  the  pig 
shows  that,  in  that  form  at  least,  the  branching  is  a  mono- 
podial one,  and  that  from  the  main  bronchus  as  it  elongates 
four  sets  of  secondary  outgrowths  develop,  namely,  a  strong- 
lateral,  a  dorsal,  a  ventral,  and  a  weak  and  variable  medial 
set. 

There  is  a  general  tendency  for  the  individual  branches 
of  the  various  sets  to  be  arranged  in  regular  succession  and 
for  their  development  to  be  symmetrical  in  the  two  lungs. 
But  on  account  of  the  necessity  under  which  the  lungs  are 
placed  of  adapting  themselves  to  the  neighboring  structures 
and  at  the  same  time  affording  a  respiratory  surface  as  large 
as  possible,  an  amount  of  asymmetry  supervenes.  Thus,  it 
has  already  been  noted  that  in  the  earliest  branching  a 
single  lateral  bronchus  is  formed  in  the  left  lung  and  two 
in  the  right.  The  uppermost  of  these  latter,  the  first  lateral 
bronchus,  is  unrepresented  in  the  left  lung,  and  is  peculiar 
in  that  it  lies  behind  the  right  pulmonary  artery  (Fig.  196, 
C),  or  in  the  adult,  after  the  recession  of  the  heart,  above  it, 
whence  it  is  termed  the  cparterial  bronchus.  Its  absence  on 
the  left  side  is  perhaps  due  to  its  suppression  to  permit  the 
normal  recession  of  the  aortic  arch  (Flint). 

So,  too,  the  inclination  of  the  heart  causes  a  suppression 
of  the  second  ventral  bronchus  in  the  left  lung,  but  at  the 
same  time  it  affords  opportunity  for  an  excessive  develop- 
ment of  the  corresponding  bronchus  of  the  right  lung,  which 
pushes  its  way  between  the  heart  and  the  diaphragm  and 
is  known  as  the  infra-cardiac  bronchus. 

As  soon  as  the  unpaired  first  lateral  1)ronchus  and  the 


THE    LUNGS. 


355 


paired  second  lateral  bronchi  are  formed  mesenchyme  begins 
to  collect  around  each  of  them  and  also  around  the  main 
bronchi,  the  lobes  of  the  adult  lung,  three  in  the  right  lung 
and  two  in  the  left,  being-  thus  outlined.  A  development  of 
mesenchyme  also  takes  place  around  the  -excessively  devel- 
oped right  second  ventral  bronchus,  and  sometimes  produces 
a  well-marked  infra-cardiac  lobe  in  the  right  lung. 

In  later  stages  the  various  bronchi  of  each  lobe  give  rise 
to  additional  branches  and  these 
again  to  others,  and  the  mesen- 
chyme of  each  lobe  grows  in 
between  the  various  branches. 
At  first  the  amount  of  mesen- 
chyme separating  the  branches  is 
comparatively  great,  but  as  the 
branches  continue,  the  growth  of 
the  mesenchyme  fails  to  keep 
pace  with  it,  so  that  in  later  stages 
the  terminal  enlargements  are 
separated  from  one  another  by 
only  very  thin  partitions  of  mes- 
enchyme, in  which  the  pulmonary 
vessels    form    a    dense    network. 


The  final  branchings  of  each  ulti- 


FiG.  197. — Diagram  of  the 
Final  Branches  of  the 
Mammalian   Bronchi. 

A,  Atrium ;  B,  bronchus ;  S, 
air-sac. —  {Miller. ) 


mate  bronchus  or  bronchiole  re- 
sults in  the  formation  at  its  ex- 
tremity of  from  three  to  five 
enlargements,  the  atria  (Fig.  197, 
A),  from  which  arise  a  number  of  air-sacs  {S)  whose  walls 
are  pouched  out  into  slight  diverticula,  the  air-cells  or  alveoli. 
Such  a  combination  of  atria,  air-sacs,  and  air-cells  consti- 
tutes a  lobule,  and  each  lung  is  composed  of  a  large  number 
of  such  units. 

The  greater  part  of  the  original  pulmonary  groove  be- 


356 


THE   LARYNX. 


comes  converted  into  the  trachea,  and  in  the  mesenchyme 
surrounding  it  the  incomplete  cartilaginous  rings  develop  at 
about  the  eighth  or  ninth  week.  The  cells  of  the  epithelial 
lining  of  the  trachea  and  bronchi  remain  columnar  or  cubi- 
cal in  form  and  become  ciliated  at  about  the  fourth  month, 
but  those  of  the  epithelium  of  the  air-sacs  become  greatly 
flattened  and  constitute  an  exceedingly  thin  layer  of  pave- 
ment epithelium. 

The  Development  of  the  Larynx. — The  opening  of  the 
upper  end  of  the  pulmonary  groove  into  the  pharynx  is 
situated  at  first  just  behind  the  fourth  branchial  furrow 
and  is  surrounded  anteriorly  and  laterally  by  the  fl  -shaped 

r  i  d  g  e  already  described 
(p.  312)  as  the  furcula, 
this  separating  it  from  the 
posterior  portion  of  the 
tongue  (Fig,  171).  The 
anterior  portion  of  this 
ridge,  which  is  apparently 
derived  from  the  ventral 
portions  of  the  third  bran- 
chial arch,  gradually  in- 
creases in  height  and  forms 

T,  o    T)  the    epiglottis,    while    the 

Fig.    198. — Reconstruction    of    the  . 

Opening  into  the  Larynx  in  an  lateral  portions,  which  pass 

Embryo   of   Twenty-eight   Days,  ,      •     1       •    ,       ,1        _ 

Seen    from    Behind   and   Above,  posteriorly    mto    the    mar- 

THE  Dorsal  Wall  OF  THE  Pharynx  q-jns     of     the     pulmonary 
BEING  Cut  Away. 

CO,    Cornicular,    and    cu,    cuneiform  gl'OOVe,     form    the    aryepi- 

tubercle;  £/),  epiglottis;  r,  unpaired  alottic     folds.      When     the 
portion   of  the  tongue. —  (Kallitfs.) 

pulmonary  groove  sepa- 
rates from  the  oesophagus,  the  opening  of  the  trachea  into 
the  pharynx  is  somewhat  slit-like  and  is  bounded  laterally 
by  the  aryepiglottic  folds,  whose  margins  present  two  ele- 
vations which  may  be  termed  the  cornicular  and  cuneiform 


THE    LARYNX.  357 

tubercles  (Fig.  198,  co  and  cu,  and  Fig.  168).  The  open- 
ing is,  however,  for  a  time,  almost  obliterated  by  a  thick- 
ening of  the  epithelium  covering  the  ridges,  and  it  is  not 
until  the  tenth  or  eleventh  week  of  development  that  it  is 
re-established.  Later  than  this,  at  the  middle  of  the  fourth 
month,  a  linear  depression  makes  its  appearance  on  the 
mesial  wall  of  each  arytenoid  ridge,  forming  the  beginning 
of  the  ventricle,  and  although  at  first  the  depression  lies 
horizontally  its  lateral  edge  later  bends  anteriorly,  so  that 
its  surfaces  look  outwards  and  inwards.     The  lips  which 


Fig.  199. — Reconstruction  of  the  Mesenchyme  Condensations  which 

Represent  the  Hyoid  and  Thyreoid  Cartilages  in  an   Embryo 

of  Forty  Days. 
The   darkly   shaded    areas    represent   centers   of   chondrification.     c.ma, 

Greater  cornu  of  hyoid ;  c.mi,  lesser  cornu ;  Th,  thyreoid  cartilage. 

—  (Kallius.) 

bound  the  opening  of  the  ventricle  into  the  laryngeal  cavity 
give  rise  to  the  ventricular  and  vocal  folds. 

The  cartilages  of  the  larynx  can  be  distinguished  during 
the  seventh  week  as  condensations  of  mesenchyme  which 
are  but  indistinctly  separated  from  one  another.  The  thy- 
reoid cartilage  is  represented  at  this  stage  by  two  lateral 
plates  of  mesenchyme,  separated  from  one  another  both 
ventrally  and  dorsally,  and  each  of  these  plates  undergoes 
chondrification  from  two  separate  centers  ( Fig.  1 99) .  These, 
as  they  increase  in  size,  unite  together  and  send  prolonga- 
tions ventrally  which  meet  in  the  mid-ventral  line  with  the 


35^  THE   LARYNXl' 

corresponding  prolongations  of  the  plates  of  the  opposite 
side,  so  as  to  enclose  an  area  of  mesenchyme  into  which  the 
chondrification  only  extends  at  a  later  period,  and  occasion- 
ally fails  to  so  extend,  producing  what  is  termed  a  foramen 
thyreoideum. 

The  mesenchymal  condensations  which  represent  the  cri- 
coid and  arytenoid  cartilages  are  continuous,  but  each  ar3^te- 
noid  has  a  distinct  center  of  chondrification,  while  the  car- 
tilage of  the  cricoid  appears  as  a  single  ring  which  is  at  first 
open  dorsally  and  only  later  becomes  complete.  The  epi- 
glottis cartilage  resembles  the  thyreoid  in  being  formed  by 
the  fusion  of  two  originally  distinct  cartilages,  from  each  of 
which  a  portion  separates  to  form  the  cuneiform  cartilages 
{cartilages  of  Wrisberg) ,  while  the  corniculate  cartilag'es 
{cartilages  of  Santorini)  are  formed  by  the  separation  of  a 
small  portion  of  cartilage  from  each  arytenoid. 

The  formation  of  the  thyreoid  cartilage  by  the  fusion  of 
two  pairs  of  lateral  elements  finds  an  explanation  from  tlie 
study  of  the  comparative  anatomy  of  the  larynx.  In  the 
lowest  group  of  the  mammalia,  the  Monotremata,  the  four 
cartilages  do  not  fuse  together  and  are  very  evidently  serially 
homologous  with  the  cartilages  which  form  the  cornua  of 
the  hyoid.  In  other  words,  the  thyreoid  results  from  the 
fusion  of  the  fourth  and  fifth  branchial  cartilages.  The 
cricoid,  in  its  development,  presents  such  striking  similari- 
ties to  the  cartilaginous  rings  of  the  trachea  that  it  is  prob- 
ably to  be  regarded  as  the  uppermost  cartilage  of  that  series, 
but  the  epiglottis  seems  to  be  a  secondary  chondrification  in 
the  glosso-laryngeal  fold  (Schaffer).  The  arytenoids  pos- 
sibly represent  an  additional  pair  of  branchial  cartilages, 
such  as  occur  in  the  lower  vertebrates  (Gegenbaur). 

These  last  arches  have  undergone  almost  complete  re- 
duction in  the  mammalia,  the  cartilages  being  their  only 
representatives,  but,  in  addition  to  the  cartilages,  the  fourth 


LITERATURE.  359 

and  fifth  arches  have  also  preserved  a  portion  of  their  mus- 
culature, part  of  which  becomes  transformed  into  the  mus- 
cles of  the  larynx.  Since  the  nerve  which  corresponds  to 
these  arches  is  the  vagus,  the  supply  of  the  larynx  is  derived 
from  that  nerve,  the  superior  laryng-eal  nerve  probably  cor- 
responding to  the  fourth  arch,  while  the  inferior  (recurrent) 
answers  to  the  fifth. 

The  course  of  the  recurrent  nerve  finds  Its  explanation  in 
the  relation  of  the  nerve  to  the  fourth  branchial  artery.  When 
the  heart  occupies  its  primary  position  ventral  to  the  floor  of 
the  pharynx,  the  inferior  laryngeal  nerve  passes  transversely 
inward  to  the  larynx  beneath  the  fourth  branchial  artery.  As 
the  heart  recedes  the  nerve  is  caught  by  the  vessel  and  is  car- 
ried back  with  it,  the  portion  of  the  vagus  between  it  and  the 
superior  laryngeal  nerve  elongating  until  the  origins  of  the  two 
laryngeal  nerves  are  separated  by  the  entire  length  of  the  neck. 
Hence  it  is  that  the  right  recurrent  nerve  bends  upward  behind 
the  right  subclavian  artery,  while  the  left  curves  beneath  the 
arch  of  the  aorta  (see  Fig.  143). 

LITERATURE. 

J.  M.  Flint:  "The  Development  of  the  Lungs,"  Amcr.  Journ.  Anat., 

VI,   1906. 
E.   GoppERT :     "  Ueber    die    Herkunft    der    Wrisbergschen    Knorpels," 

Morphol.  Jahrhuch,  xxi,  1894. 
W.  His:  "Zur  Bildungsgeschichte  der  Lungen  beim  menschlichen»Em- 

bryo,"  Archiv  filr  Anat.  unci  Physiol.,  Anat.  Ahth.,  1887. 
E.   Kallius  :   "  Beitrage  zur  Entwickelungsgeschichte  des   Kehlkopfes," 

Anat.  Hefte,  ix,  1897. 
E.  Kallius*:  "  Die  Entwickelung  des  menschlichen  Kehlkopfes,"   Ver- 

handl.  der  Anat.  Gesellsch.,  xii,   1898. 
A.  Narath  :  "  Der  Bronchialbaum  der  Saugethiere  und  des  Menschen," 

Bibliotheca  Medica,  Abfh.  A,  Heft  3,  1901. 
J.   Schaffer:   "Zur  Histologic   Histogenese   und  phylogenetischen   Be- 

deutung  der  Epiglottis,"  Anat.  Hcftc.  xxxiii,  1907. 


CHAPTER  XIII. 

THE    DEVELOPMENT    OF    THE    URINOGENITAL 

SYSTEM. 

The  excretory  and  reproductive  systems  of  organs  are 
so  closely  related  in  their  development  that  they  must  be 
considered  together.  They  both  owe  their  origin  to  the 
mesoderm  which  constitutes  the  intermediate  cell-mass 
(p.  104),  this,  at  an  early  period  of  development,  becom- 
ing thickened  so  as  to   form  a  ridge  projecting  into  the 


.CL 


0  ■=:!^ 


J    L 


^    y'^  ni         ^' 

Fig.  200. — Transverse  Section  through  the  Abdominal  Region  of 

A  Rabbit  Embryo  of  12  mm. 
a,  Aorta;   gl.,  glomerulus;  gr,  genital   ridge;   m,  mesentery;   nc,  noto- 

chord ;  t,  tubule  of  mesonephros ;  wd,  Wolffian  duct ;  ivr,  Wolffian 

x\A%^.—  {Miha\kovicz.') 

dorsal  portion  of  the  coelom  and  forming  what  is  known 
as  the  Wolffian  ridge  (Fig.  200,  2vr).  The  greater  portion 
of  the  substance  of  this  ridge  is  concerned  in  the  develop- 
ment of  the  primary  and  secondary  excretory  organs,  but 
on  its  mesial  surface  a  second  ridge  appears  which  is  des- 
tined to  give  rise  to  the  ovary  or  testis,  and  hence  is  termed 

the  genital  ridge  (gr). 

360 


THE    PRONEPHROS.  36 1 

The  development  of  the  excretory  organs  is  remarkable 
in  that  three  sets  of  organs  appear  in  succession.  The 
first  of  these,  the  pronephros,  exists  in  a  very  rudimentary 
condition  in  the  human  embryo,  although  its  duct,  the 
pronephric  or  Wolffian  duct,  undergoes  complete  develop- 
ment and  plays  an  important  part  in  the  development  of 
the  succeeding  organs  of  excretion  and  also  in  that  of  the 
reproductive  organs.  The  second  set,  the  mesonephros  or 
WoMan  body,  reaches  a  considerable  development  during 
embryonic  life,  but  later,  on  the  development  of  the  final 
set,  the  definite  kidney  or  metanephros,  undergoes  degen- 
eration, portions  only  persisting  as  rudimentary  structures 
associated  for  the  most  part  with  the  reproductive  organs. 

The  Development  of  the  Pronephros  and  the  Pro- 
nephric Duct. — The  first  portions  of  the  excretory  system 
to  make  their  appearance  are  the  pronephric  or  Wolffian 
ducts,  and  these  develop  as  thickenings  of  the  lateral  parts 
of  the  intermediate  cell-masses.  At  first  the  thickenings 
form  soHd  cords  of  cells  (Fig.  201,  ivd),  but  later  a  lumen 
appears  in  the  center  of  each  cord,  which  thus  becomes 
converted  into  a  canal.  In  early  stages  the  cords,  toward 
their  posterior  ends,  may  undergo  a  secondary  fusion  with 
the  immediately  overlying  ectoderm  (Martin)  and  may 
thereby  present  the  appearance  of  having  arisen  from  that 
layer,  but  when  fully  developed  the  ducts  lie  in  the  sub- 
stance of  the  Wolffian  ridges  (Fig.  200,  zvd),  their  anterior 
ends  being  situated  well  forward  in  the  region  occupied  by 
the  heart,  whence  they  extend  backward  to  open  on  the 
ventral  part  of  the  lateral  walls  of  the  cloaca  (Fig.  163). 

The  pronephros  has  been  observed  in  embryos  of  about 
3  mm.  as  two  tubular  invaginations  of  the  coelomic  epi- 
thelium into  the  substance  of  each  Wolffian  ridge,  in  the 
region  in  which  the  anterior  end  of  the  Wolffian  duct  is 
found  (Janhosik).  The  tubules  do  not  proceed  to  com- 
32 


362 


THE    PRONEPHROS. 


plete  development,  making  no  connection  with  the  duct,  and 
indeed  the  anterior  one  hardly  deserves  to  be  termed  a 
tubule,  since  it  is  a  solid  cord  of  cells,  continuous  at  one 
extremity  with  the  coelomic  epithelium.  The  posterior  one 
is,  however,  a  hollow  tubule  ending  blindly  at  one  extrem- 
ity, while  at  the  other  it  communicates  with  the  coelomic 
cavity,  the  opening  being  termed  a  nephrostome.  Oppo- 
site these  rudimentary  tubules  there  arises  from  the  root 
of  the  mesentery  a  process  which  projects  freely  into  the 
coelom  toward  the  nephrostomes.     This  probably  represents 


im 


iic 


en 


Fig. 


201. — Transverse    Section   through    Chick    Embryo   of   about 
Thirty-six  Hours. 
en,   Endoderm;  im,   intermediate  cell  mass;   wis,  mesodermic  somite; 
nc,   notochord;    so,   somatic,    and    sp,   splanchnic   mesoderm;    wd, 
Woliifian  duct. — (Waldeyer.) 


a  rudimentary  free  glomerulus,  into  which  branches  from 

the  aorta  may  project. 

Structures  which  are  probably  to  be  identified  as  pro- 

nephric  tubules  have  been  observed  in  older  embryos  up  to 

20  mm.  (Tandler),  situated  at  the  sides  of  the  aorta  from 

about  the  seventh  to  the  eleventh  segments.     They  present, 

however,  signs  of  degeneration,  having,   for  instance,  no 

connection  with  the  coelomic  epithelium  in  older  embryos, 

and  it  seems  probable  that  they  later  disappear  completely. 

A  similar  but  more  perfectly  developed  pronephros  has  been 
described  in  other  mammals,  such  as  the  rabbit  and  rat,  and  is 
of  constant  occurrence  in  all  the  lower  vertebrates.  In  these 
the  pronephric  tubules,  which  may  be  six  (in  the  lamprey)  or 


THE    PRONEPHROS.  363 

more  in  number  on  each  side,  are  primarily  arranged  segment- 
ally,  and  open  by  one  extremity  into  the  anterior  portion  of  the 
Wolffian  duct  and  by  the  other  into  the  coelomic  cavity,  and, 
furthermore,  each  tubule  has  corresponding  to  it  a  glomerulus 
which  lies  freely  in  the  ccelomic  cavity  in  the  vicinity  of  the 
nephrostome.  By  these  free  glomeruli  and  by  the  possession 
of  nephrostomes  the  tubules  of  the  pronephros  are  distin- 
guished from  those  of  the  mesonephros  in  the  higher  verte- 
brates, and  since  both  these  peculiarities  are  represented  in  the 
two  pairs  of  tubules  described  above  as  occurring  in  the  3  mm. 
human  embryo,  there  seems  to  be  little  room  for  doubt  but 
that  they  are  representatives  of  a  rudimentary  pronephros. 

It  has  been  very  generally  supposed  that  the  tubules  of  the 
mesonephros,  which  develop  in  the  segments  succeeding  those 
which  contain  the  pronephros,  were  serially  homologous  with 
the  pronephric  tubules.  Doubts  have  recently  been  aroused 
against  this  theory  (Riickert,  Wheeler).  Important  structural 
differences  exist  in  the  two  sets  of  tubules,  and  since  even  in 
the  lowest  vertebrates  the  pronephros  seems  to  be  a  rudimen- 
tary structure,  it  has  been  held  not  improbable  that  in  the 
ancestors  of  the  vertebrates  it  was  a  much  more  perfectly 
developed  organ  extending  back  into  the  region  occupied  by 
the  mesonephros  in  existing  vertebrates.  As  the  mesonephros 
developed  the  pronephros  underwent  degeneration,  portions  of 
its  tubules  persisting,  however,  and  uniting  to  form  a  continu- 
ous canal,  the  pronephric  duct,  a  structure  for  which,  other- 
wise, it  is  difficult  to  find  a  satisfactory  explanation.  The  fact 
that  in  lower  forms  the  duct  seems  to  develop  as  a  number  of 
separate  parts  which  later  become  continuous  stands  in  favor 
of  this  hypothesis,  but  in  opposition  to  it  is  the  observation 
that  the  lower  portion  of  the  duct  in  several  species  of  mam- 
mals arises  from  the  ectoderm  (von  Spec,  Flemming).  It 
seems,  however,  to  be  established  that  in  the  majority  of  the 
lower  vertebrates  it  is  of  purely  mesodermal  origin,  and  its 
connection  with  the  ectoderm  in  the  mammalia  is  therefore 
very  probably  due  to  a  secondary  fusion  (Martin). 

The  Development  of  the  Mesonephros.-^The  pro- 
nephric duct  does  not  disappear  with  the  degeneration  of 
the  pronephric  tubules,  but  persists  to  serve  as  the  duct  for 
the  mesonephros  and  to  play  an  important  part  in  the  devel- 
opment of  the  metanephros  also.  In  the  Wolffian  ridge 
there  appear  in  embryos  of  between  3  and  4  mm.  a  num- 


304 


THE    MESONEPHROS. 


ber  of  coiled  tubules,  which  arise  by  some  of  the  cells  of 
the  ridge  aggregating  to  form  solid  cords,  at  first  entirely 
unconnected  with  either  the  coelomic  epithelium  or  the 
Wolffian  duct.  Later  the  cords  become  connected  with  the 
coelomic  epithelium  and  acquire  a  lumen,  and  near  the 
coelomic  end  of  the  tubule  a  condensation  of  the  mesen- 
chyme of  the  Wolffian  ridge  occurs  to  form  a  glomerulus 

into  which  a  branch  ex- 
tends from  the  neighbor- 
ing aorta.  The  tubules 
finally  acquire  connection 
with  the  Wolffian  duct 
and  at  the  same  time  lose 
their  connections  with  the 
coelomic  epithelium,  their 
nephrostomes  being  ac- 
cordingly but  transitory 
structures.  The  tubules 
rapidly  increase  in  length 
and  become  coiled,  and 
the  glomeruli  project  into 
their  cavities,  pushing  in 
front  of  them  the  wall  of 
the  tubule  so  that  it  has  the  appearance  represented  in  Fig 

202. 

It  seems  probable  that  primarily  the  mesonephric  cords 
are  arranged  segmentally,  a  single  pair  occurring  in  each 
segment  of  the  body  behind  the  pronephros  as  far  back, 
probably,  as  the  pelvic  region,  and  hence  the  intermediate 
cell-mass  from  which  the  Wolffian  ridge  is  formed  may 
properly  be  regarded  as  composed  of  nephrotomes,  even 
though  no  surface  indications  of  segmentation  are  to  be 
seen  in  it.  The  correspondence  of  the  tubules  with  the 
myotomes  becomes,  however,  early  disturbed,  partly  as  the 


■"^m-es 


Fig.  202. — Transverse  Section  of 
THE  Wolffian  Ridge  of  a  Chick 
Embryo  of  Three  Days. 

ao.  Aorta;  gl,  glomerulus;  gr,  genital 
ridge;  mes,  mesentery;  mt,  meso- 
nephric tubule ;  vc,  cardinal  vein ; 
Wd,  Wolffian  duct. — {Mihalkovicz.) 


THE    MESONEPHROS. 


365 


result  of  differences  in  growth  of  the  two  structures,  but 
especially  because  a  number  of  secondary  and  tertiary  tu- 
bules develop  in  connection  with  each  of  the  primary  ones. 
Exactly  how  these  additional  tubules  arise  is  a  little  uncer- 
tain, some  observers  maintaining  that  they  are  formed  from 
the  substance  of  the  Wolffian  ridge  in  the  same  manner  as 
the  primary  tubules  with  which  they  later  become  con- 
nected   (Mihalkovicz),    while    others    hold    that    they    are 


.^.0^' 


-ao 


Fig.  203.— Urinogenital  Apparatus  of  a  Male  Pig  Embryo  of  6  cm. 
ao,    Aorta;     h,    bladder;    gh,    gubernaculum    testis;     k,    kidney;    md, 

Mullerian  duct;  sr,  suprarenal  body;  t,  testis;  w,  Wolffian  body; 

wd,  Wolffian  duct  (Mihalkovics.) 

formed  by  the  splitting  of  the  primary  tubules  or  as  buds 
from  these  (Braun,  Janhosik). 

By  the  formation  of  these  additional  tubules  and  the 
continued  elongation  of  all,  whereby  they  become  thrown 
into  numerous  convolutions,  the  Wolffian  ridge  becomes  a 


366 


THE    METANEPHROS. 


somewhat  voluminous  structure,  projecting  markedly  into 
the  coelomic  cavity  (Fig.  203).  It  is  attached  to  the  dorsal 
wall  of  the  body  by  a  distinct  mesentery  and  has  in  its 
lateral  portion,  embedded  in  its  substance,  the  Wolffian 
duct,  while  on  its  mesial  surface  anteriorly  is  the  but  slightly 
developed  genital  ridge  (t).  This  condition  is  reached  in 
the  human  embryo  at  about  the  sixth  or  seventh  week  of 
development,  and  after  that  period  the  mesonephros  under- 
goes rapid  degeneration,  so  that  at  about  the  sixteenth  week 
nothing  remains  of  it  except  the  duct  and  a  few  small  rudi- 
ments whose  history  will  be  given  later. 

The  Development  of  the  Metanephros. — The  first  por- 
tion of  the  metanephros  to  appear  is  a  tubular  outgrowth 

from  the  dorsal  surface 
of  the  Wolffian  duct, 
shortly  before  its  en- 
trance into  the  cloaca 
(Fig.  163).  As  this 
outgrowth  elongates  it 
comes  to  lie  dorsal  to 
the  mesonephros  and 
its  anterior  extremity 
becomes  enlarged  and 
lobed,  and  also  becomes 
surrounded  by  a  conden- 
sation of  mesenchyme, 
which  has  been  termed 
the  metanephric  blastema.  The  outgrowth  makes  its  ap- 
pearance in  embryos  of  about  5  mm.,  but  its  anterior  ex- 
tremity does  not  reach  its  final  position  in  the  neighborhood 
of  the  suprarenal  body  until  the  third  month  of  development. 
The  extremity  of  the  outgrowth  early  begins  to  divide 
within  the  substance  of  the  blastema  and  thus  gives  rise 
to  a  number  of  branches,  each  of  which  terminates  in  an 


Fig.  204. — Diagrams  of  Early  Stages 
IN  THE  Development  of  the  Meta- 
nephric Tubules. 

t,  Urinary  tubule;  Ur,  ureter;  v,  renal 
ampulla. —  {Haycraft. ) 


THE    METANEPHROS. 


1^7 


ampullar  enlargement,  lying  in  the  cortical  portion  of  the 
blastema  (Fig.  204),  which  by  this  time  has  formed  for 
itself  a  capsule.  In  the  vicinity  of  each  ampulla  a  number 
of  condensations  of  the  blastemic  tissue  occur  (Fig.  205, 
A),  forming  renal  vesicles  which  are  at  first  solid  but  later 
become  hollow,  and  each  of  these  elongates  to  form  an 
S-shaped  tubule,  one  end  of  which  becomes  continuous  with 
the  neighboring  ampulla  (Figs.  204,  B,  and  205,  B).  In 
the  space  enclosed  by  what  may  be  termed  the  lower  loop 


Fig.  205. — Four  Stages  in  the  Development  of  a  Uriniferous 
Tubule,  of   a    Cat. 

A,  arched  collecting  tubule,  C,  pfokinial  convoluted  tubule ;  C,  distal 
convoluted  tubule ;  H,  loop  of  HenlCi;  M,  glomerulus ;  T,  renal 
vesicle;  V,  ampulla  (drawn  from  reconstructions  prepared  by  G. 
C.    Huber). 

of  the  S  a  collection  of  mesenchyme  cells  appears,  and  into 
this  branches  penetrate  at  an  early  stage  from  the  renal 
artery  to  form  a  glomerulus,  the  neighboring  walls  of  the 
tubule  becoming  exceedingly  thin  and  being  transformed 
into  a  capsule  of  Bowman.     The  upper  loop  of  the  S  now 


5^8  THE    METANEPHROS. 

begins  to  elongate  (Fig.  205,  C),  growing  toward  the  liilus 
of  the  kidney,  parallel  to  the  branch  of  the  outgrowth  from 
the  Wolffian  duct  to  which  it  is  attached  and  between  this 
and  the  glomerulus,  and  forms  a  loop  of  Henle.  From  the 
portion  of  the  horizontal  limb  of  the  S  which  lies  between 
the  glomerulus  and  the  descending  limb  of  the  loop  of  Henle 
the  proximal  convoluted  tubule  (C)  arises,  while  the  distal 
convoluted  and  the  arched  collecting  tubules  (C  and  A)  are 
formed  from  the  uppermost  portion  of  the  upper  loop  (Fig. 
205,  D). 

The  entire  length  of  the  urinary  tubule  from  Bowman's 
capsule  to  the  arched  collecting  tube  inclusive  is  thus  de- 
rived from  a  renal  vesicle,  while  each  renal  ampulla  becomes 
by  its  elongation  a  collecting  tubule  and  the  remaining  por- 
tions of  the  outgrowth  from  the  Wolffian  duct  become  the 
pelvis  of  the  kidney  and  the  ureter.  Up  to  the  time  when 
the  urinary  tubules  begin  to  develop  there  is  no  pelvis  to  the 
kidney,  the  ureter  extending  well  toward  the  center  of  the 
blastema  before  beginning  to  branch  and  the  branches 
thence  extending  to  the  cortex  (Fig.  204).  As  soon  as 
the  tubules  appear,  however,  the  formation  of  the  pelvis 
begins  by  what  has  been  described  as  an  evagination  of  the 
primary  branches  of  the  ureter  to  form  a  common  cavity, 
a  process  which  is  beginning  to  manifest  itself  in  the  stage 
shown  in  Fig.  204,  B,  and  which  is  continued  until  the 
secondary  branches  are  also  taken  up  into  the  cavity,  into 
which  the  various  collecting  tubules  then  open  separately. 

At  about  the  tenth  week  of  development  the  surface  of 
the  human  kidney  becomes  marked  by  shallow  depressions 
into  lobes,  of  which  there  are  about  eighteen,  one  corre- 
sponding to  each  of  the  groups  of  tubules  which  arise  from 
the  same  renal  vesicle.  This  lobation  persists  until  after 
birth  and  then  disappears  completely,  the  surface  of  the 
kidney  becoming  smooth. 


THE    MULLERIAN    DUCT.  369 

The  Developrnent  of  the  Miillerian  Duct  and  of  the 
Genital  Ridge. — At  the  time  when  the  Wolffian  body  has 
ahnost  reached  its  greatest  development  a  second  longitu- 
dinal duct  makes  its  appearance  in  close  proximity  to  the 
Wolffian.  This  is  known  as  the  Miillerian  duct  (Fig.  206, 
Md).  Its  development  is  preceded  by  the  appearance  of 
a  distinct  ridge  or  fold  upon  the  ventral  surface  of  the 
Wolffian  body,  extending  from  the  under  surface  of  the 
diaphragm  above  to  the  urogenital  sinus  below  and  con- 
taining in  the  lower  portion  of  its  course  the  Wolffian  duct 
(Fig.  203).  Near  the  anterior  end  of  the  mesonephros 
there  grows  into  this  fold  an  evagination  from  the  peri- 
toneum covering  the  Wolffian  ridge  and  by  the  proliferation 
of  the  cells  at  its  tip  this  evagination  gradually  extends 
downward  in  the  substance  of  the  ridge,  and  in  embryos 
of  22  mm.  has  reached  the  urogenital  sinus.  As  they  ap- 
proach the  sinus,  the  right  and  left  evaginations  or  Miil- 
lerian ducts  gradually  approach  one  another  and  finally  fuse 
together  to  form  a  single  tube  in  the  lower  part  of  their 
course,  but  they  remain  distinct  above,  each  tube  retaining 
its  original  opening  into  the  peritoneal  cavity. 

The  genital  ridge  makes  its  appearance  as  a  band-like 
thickening  of  epithelium  extending  lengthwise  upon  the 
mesial  surface  of  the  Wolffian  body.  The  cells  composing 
the  thickening  are  arranged  in  several  layers  and  are  of 
two  kinds :  ( i )  smaller,  cubical  or  spherical  epithelial  cells, 
with  a  relatively  small  amount  of  cytoplasm,  and  (2)  large 
spherical  cells  with  more  abundant  and  clear  cytoplasm, 
known  as  sex-cells.  Later  the  thickening  separates  into 
two  distinct  layers,  a  superficial  one  which  remains  epi- 
thelial in  character  and  contains  the  sex-cells,  and  a  deeper 
one,  composed  only  of  smaller  cells  and  known  as  the  stroma 
layer,  since  its  cells  later  become  the  stroma  cells. 

From  the  epithelial  layer  columns  of  cells  containing  sex- 


370 


THE   GENITAL    RIDGE. 


N 


Fig.  206. — Transverse  Section  through  the  Abdominal  Region  of 
AN   Embryo  of  25    mm. 

Ao,  Aorta;  B,  bladder;  I,  intestine;  L,  liver;  M,  muscle;  Md,  Miillerian 
duct ;  A'',  spinal  cord ;  Ov,  ovary ;  RA,  rectus  abdominis ;  Sg,  spinal 
ganglion;  UA,  umbilical  artery;  Ur,  ureter;  V,  vertebra;  W ,  Wolf- 
fian body;  Wd,  Wolffian  duct. —  (Keibel.) 


THE    GENITAL    RIDGE.  3/1 

cells  grow  down  into  the  subjacent  tissue  of  the  ridge,  this 
process  not  taking  place,  however,  to  an  equal  extent  in  all 
portions  of  the  ridge.  Indeed,  three  regions  may  be  recog- 
nized in  the  ridge;  an  anterior  one,  in  which  a  relatively 
small  numiber  of  cell-columns,  extending  deeply  into  the 
stroma,  are  formed;  a  middle  one  in  which  numerous  col- 
umns are  formed;  and  a  posterior  one  in  which  practically 
none  are  formed.  The  first  region  has  been  termed  the 
rete  region  and  its  cell-columns  the  rete-cords,  the  second 
region  the  sex-gland  region  and  its  columns  the  sex-cords, 
and  the  posterior  region  is  the  mesenteric  region  and  plays 
no  part  in  the  actual  formation  of  the  ovary  or  testis. 

The  histological  differentiation  of  the  genital  ridge  de- 
scribed above  is  common  to  both  sexes  and,  in  the  human 
embryo,  is  completed  at  about  the  fourth  or  fifth  week. 
After  that  period  the  development  differs  somewhat  accord- 
ing as  the  sex-gland  is  to  become  a  testis  or  an  ovary,  and 
consequently  the  further  development  of  these  two  struc- 
tures may  be  described  separately.  Before  doing  so,  how- 
ever, the  development  of  certain  accessory  structures  may 
be  briefly  described. 

At  first  the  ridge  is  of  insignificant  dimensions  compared 
with  the  more  voluminous  Wolffian  body  (Fig.  200),  but 
as  the  degeneration  of  the  latter  proceeds  the  relative  size 
of  the  two  structures  becomes  reversed  and  the  genital  ridge 
forms  a  marked  prominence  attached  to  the  surface  of  the 
Wolffian  ridge  by  a  fold  of  peritoneum  which  becomes  the 
mesovarium  in  the  female  and  the  mesorchium  in  the  male. 
The  fold  which  surrounds  the  Wolffian  body  becomes  trans- 
formed on  the  degeneration  of  that  structure  into  the  broad 
ligament,  the  transverse  position  of  which  in  the  adult  is  due 
to  the  fusion  of  the  lower  portions  of  the  Miillerian  ducts, 
and  since  the  genital  ridges  lie  primarily  to  the  median  side 
of  the  ducts,  they  come  to  be  attached  by  their  mesentery 


372 


THE    TESTIS. 


to  the  dorsal  surface  of  the  broad  ligament.  The  relations 
of  the  broad  ligaments  and  mesorchia  in  the  male  become 
profoundly  modified  by  the  descent  of  the  testes  into  the 
scrotum,  a  process  to  be  described  later  (p.  388).  From 
each  genital  ridge  a  prolongation  of  mesenchyme  extends 

Ff]  downward  in  the  mesentery 
of  the  ridge,  nearly  parallel 
with  the  Miillerian  duct,  with 
which  it  comes  into  contact 
at  the  point  where  the  two 
ducts  fuse  and  thence  is  con- 
tinued downward  and  for- 
ward between  the  folds  of  the 
broad  ligament  to  be  attached 
to  the  ventral  wall  of  the 
abdomen  in  the  inguinal  region. 
The  upper  part  of  this  pro- 
longation of  the  genital  ridge 
represents  the  ligament  of  the 
ovary  and  its  lower  part  the 
ligamentum  teres  of  the  female 
(Fig.  207),  while  in  the  male 
the  entire  structure  forms  what 
is  known  as  the  gnhernaculum 
testis. 
The  Development  of  the  Testis. — At  about  the  fourth  or 
fifth  week  there  appears  in  the  sex-gland  region  of  the 
genital  ridge  a  structure  which  serves  to  characterize  the 
region  as  a  testis.  This  is  a  layer  of  somewhat  dense  con- 
nective tissue  which  grows  in  between  the  epithelial  and 
stroma  layers  of  the  sex-gland  region  and  gradually  ex- 
tends around  the  entire  sex-gland  to  form  the  tunica  albu- 
ginea.  By  its  development  the  sex-cords  are  separated  from 
the   epithelium,   which   later  becomes   much   flattened   and 


Fig.  207. — Reproductive  Or- 
gans OF  A  Female  Embryo 
OF  Six  Months. 

B,  Bladder;  F,  Fallopian  tube; 
/,  intestine;  01,  ovarian  liga- 
ment; Ov,  ovary;  Rl,  round 
ligament;  UA,  umbilical  ar- 
tery; Ur,  ureter;  Ut,  uterus; 
W,  Wolffian  body  (epoopho- 
ron) . —  (Adapted  from  Mihal- 
kovics. ) 


THE    TESTIS. 


373 


eventually  almost  disappears.  Shortly  after  the  appearance 
of  the  albuginea  the  sex-cords  become  broken  up  into  more 
or  less  spherical  masses  and  the  rete-cords  grow  backwards 
into  the  axial  substance  of  the  testis  (Fig.  208),  develop  a 
lumen  and  send  off  branches,  one  of  which  becomes  con- 
nected with  each  of  the  masses  formed  from  the  sex-cords. 
The  rete-cords  have  also  come  into  connection  with  the 
glomeruli  of  the  anterior  portion  of  the  mesonephros  and, 
like  the  sex-cords,  have  separated  from  the  epithelium  which 


Fig.  208. — Section  through  the  Testis  and  the  Broad  Ligament  of 

THE  Testis  of  an  Embryo  of  5.5   mm. 

ep,  Epithelium;  md,  Miillerian  duct;  mo,  mesorchium ;  re,  rete-cords; 

sc,   sex-cords;   zvd.   Wolffian   duct. —  {Mihalkovics.) 

gave  rise  to  them,  so  that  they  now  extend  between  the 
sex-cord  masses  and  the  mesonephric  glomeruli.  The  sex- 
cord  masses  eventually  form  the  seminiferous  tubules,  while 
from  the  rete-cords  are  formed  the  tubuli  recti  and  rete  tes- 
tis, by  which  the  spermatozoa  are  transmitted  to  the  Wolf- 
fian duct  (see  p.  378). 

The  development  of  the  seminiferous  tubules  is  not  com- 
pleted, however,  until  puberty.     The  sex-cord  masses  elon- 


374 


THE   TESTIS. 


gate  to  form  cylindrical  cords,  between  which  lie  stroma 
cells  and  interstitial  cells  derived  from  the  stroma ;  but  until 
puberty  these  cords  remain  solid,  a  lumen  developing  only 
at  that  period.     The  cords  contain  the  same  forms  of  cells 


...,     ep 


R 


Mc    — 


Mn 


Fig.   209. — Longitudinal  Section  of  the  Ovary  of  an   Embryo  Cat 

OF  9.4   CM. 
cor,  Cortical  layer;  cp,  epoophoron  ;  Mc,  medullary  cords;  Mn,  meso- 
nephros ;   pf,  peritoneal   fold  containing  Fallopian   tube ;   R,   rete ; 
^.^  Fallopian  tube. —  (Coert,  from  Bilhler.) 


as  were  described  as  occurring  in  the  epithelium  of  the 
germinal  ridge,  and  while  in  the  early  stages  transitional 
forms  seem  to  occur,  in  later  periods  the  two  varieties  of 
cells  are  quite  distinct,  the  sex-cells  becoming  spermatogonia 


THE   OVARY.  37$ 

(see  p.  13)  and  being  the  mother  cells  of  the  spermatozoa, 
while  the  remaining  epithelial  cells  perhaps  become  trans- 
formed into  Sertoli  cells  (Benda). 

The  Development  of  the  Ovary. — In  the  case  of  the  ovary, 
after  the  formation  of  the  sex-cords,  connective  tissue  grows 
in  between  these  and  the  epithelium,  forming  a  layer  equiva- 
lent to  the  tunica  albuginea  of  the  testis.  It  is,  however,  a 
much  looser  tissue  than  its  homologue  in  the  male,  and, 
indeed,  does  not  completely  isolate  the  sex-cords  from  the 
epithelium,  although  the  majority  of  the  cords  are  separated 
and  sink  into  the  deeper  portions  of  the  ovary  where  they 
form  what  have  been  termed  the  medullary  cords.  In  the 
meantime  the  germinal  epithelium  has  continued  to  bud  off 
cords  which  unite  to  form  a  cortical  layer  of  cells  lying 
below  the  epithelium  and  separated  from  the  medullary 
cords  by  the  tunica  albuginea  (Fig.  209). 

Later  the  cortical  layer  becomes  broken  up  by  the  in- 
growth of  stroma  tissue  into  spherical  or  cord-like  masses, 
consisting  of  sex-cells  and  epithelial  cells  (Fig.  210).  The 
invasion  of  the  stroma  continuing,  these  spheres  or  cords 
{Pfliiger's  cords)  become  divided  into  smaller  masses,  the 
primary  ovarian  follicles,  each  of  which  consists  as  a  rule 
of  a  single  sex-cell  surrounded  by  a  number  of  epithelial 
cells,  the  whole  being  enclosed  by  a  zone  of  condensed 
stroma  tissue,  which  eventually  becomes  richly  vascularized 
and  forms  a  theca  folliculi  (Fig.  9).  The  epithelial  cells 
in  each  follicle  are  at  first  comparatively  few  in  number  and 
closely  surround  the  sex-cell  (Fig.  210,  /),  which  is  destined 
to  become  an  ovum,  but  in  certain  of  the  follicles  they  un- 
dergo an  increase  by  mitosis,  becoming  extremely  numerous, 
and  later  secrete  a  fluid,  the  liquor  folliculi,  which  collects 
at  one  side  of  the  follicle  and  eventually  forms  a  consider- 
able portion  of  its  contents.  The  follicular  cells  are  differ- 
entiated by  its  appearance  into  the  stratum  granulosum', 


376 


THE    OVARY. 


which  surrounds  the  wall  of  the  follicle,  and  the  discus  pro- 
ligeriis,  in  which  the  ovum  is  embedded  (Fig.  9,  dp),  and 
the  cells  which  immediately  surround  the  ovum,  becoming 
cylindrical  in  shape,  give  rise  to  the  corona  radiata  (Fig. 
10,  cr). 

A  somewhat  similar  fate  is  shared  by  the  medullary  cords, 
these  also  breaking  up  into  a  number  of  follicles,  but  sooner 
or  later  these  follicles  undergo  degeneration  so  that  shortly 

after  birth  practically 
no  traces  of  the  cords 
remain.  It  must  be 
noted  that  degeneration 
of  the  follicles  formed 
from  the  cortical  layer 
also  takes  place  even 
during  fetal  life  and 
continues  to  occur 
throughout  the  entire 
periods  of  growth  and 
Fig.  210.— Section  of  the  Ovary  of  a  functional  activity,  nu- 
New-born  Child.    _  merous    atretic    follicles 

a,  Ovarial  epithelium ;  b,  proximal  part         .  .  1   •       1 

of  a   Pfliiger's   cord;   c,  sex-cell   in  bemg  found  m  the  ovary 

epithelium;  d  and  ^,  spherical  masses;    ^^   ^^    ^j^ies.      Indeed   it 

f,  primary  follicle;   g.  blood-vessel. — 

(From  Gegenbaur,  after  Waldeyer.)  WOuld  seem  that  degen- 
eration is  the  fate  of  the 
great  majority  of  the  follicles  and  sex-cells  of  the  ovary, 
but  few  ova  coming  to  maturity  during  the  life-time  of 
any  individual. 

Rete-cords  developed  from  the  rete  portion  of  the  germ- 
inal ridge  occur  in  connection  with  the  ovary  as  well  as  with 
the  testis  and  form  a  rete  ovarii.  They  do  not,  however, 
extend  so  deeply  into  the  ovary,  remaining  in  the  neighbor- 
hood of  the  mesovarium,  and  they  do  not  become  tubular, 


THE    GENITAL   DUCTS.  377 

but  resemble  closely  the  medullary  cords  with  which  they 
are  serially  homologous.  They  separate  from  the  epithe- 
lium and  make  connections  with  the  glomeruli  of  the  ante- 
rior portion  of  the  mesonephros,  on  the  one  hand,  and  on  the 
other  with  the  medullary  cords,  and  in  later  stages  show  a 
tendency  to  break  up  into  primary  follicles,  which  early 
degenerate  and  disappear  like  those  of  the  medullary  cords. 

The  Transformation  of  the  Mesonephros  and  the 
Ducts. — At  one  period  of  development  there  are  present,  as 
representatives  of  the  urinogenital  apparatus,  the  Wolffian 
body  (mesonephros)  and  its  duct,  the  Miillerian  duct,  and 
the  developing  ovary  or  testis.  Such  a  condition  forms 
an  indifferent  stage  from  which  the  development  proceeds 
in  one  of  two  directions  according  as  the  genital  ridge  be- 
comes a  testis  or  an  ovary,  the  Wolffian  body  in  part  under- 
going degeneration  and  in  part  persisting  to  form  organs 
which  for  the  most  part  are  rudimentary,  while  in  the 
female  the  Wolffian  duct  also  degenerates  except  for  certain 
rudiments  and  in  the  male  the  Miillerian  duct  behaves 
similarly. 

In  the  Male. — It  has  been  seen  that  the  upper  portion 
of  the  Wolffian  body,  through  the  rete  cords,  enters  into 
very  intimate  relations  with  the  testis,  and  may  be  regarded 
as  divided  into  two  portions,  an  upper  genital  and  a  lower 
excretory.  In  the  male  the  genital  portion  persists  in  its 
entirety,  serving  as  the  efferent  ducts  of  the  testis,  which, 
beginning  in  the  spaces  of  the  rete  testis,  already  shown  to 
be  connected  with  the  capsules  of  Bowman,  open  into  the 
upper  part  of  the  Wolffian  duct  and  form  the  globus  major 
of  the  epididymis.  The  excretory  portion  undergoes  ex- 
tensive degeneration,  a  portion  of  it  persisting  as  a  mass 
of  coiled  tubules  ending  blindly  at  both  ends,  situated  near 
the  head  of  the  epididymis  and  known  as  the  paradidymis 
or  organ  of  Giraldes,  while  a  single  elongated  tubule,  aris- 
33 


3/8  THE    GENITAL    DUCTS, 

ing  from  the  portion  of  the  Wolffian  duct  which  forms  the 
globus  minor  of  the  epididymis,  represents  another  portion 
of  it  and  is  known  as  the  vas  aherrans. 

The  Wolffian  duct  is  retained  complete,  the  portion  of 
it  nearest  the  testis  becoming  greatly  elongated  and  thrown 
into  numerous  coils,  forming  the  body  and  globus  minor 
of  the  epididymis,  while  the  remainder  of  it  is  converted 
into  the  vas  deferens  and  the  ductus  ejaculatorius.  A  lat- 
eral outpouching  of  the  wall  of  the  duct  to  form  a  longi- 
tudinal fold  appears  at  about  the  third  month  and  gives 
rise  to  the  vesicida  seminalis,  the  lateral  position  of  the  out- 
growth explaining  the  adult  position  of  the  vesiculae  lateral 
to  the  vasa  deferentia. 

With  the  Miillerian  duct  the  case  is  very  different,  since 
it  disappears  completely  throughout  the  greater  part  of  its 
course,  only  its  upper  and  lower  ends  persisting,  the  former 
giving  rise  to  a  small  sac-like  body,  the  sessile  hydatid  of 
Morgagni,  attached  to  the  upper  end  of  the  testis  near  the 
epididymis,  while  the  latter  is  represented  by  a  depression 
in  the  floor  of  the  urethra  known  as  the  sinus  pocularis, 
which  is  usually  prolonged  upward  into  a  short  cylindrical 
pouch  known  as  the  litems  masculinus,  although  it  corre- 
sponds to  the  vagina  rather  than  to  the  uterus  of  the  female. 

In  the  Female. — In  the  female  the  genital  portion  of  the 
mesonephros,  though  never  functional  as  ducts,  persists  as 
a  group  of  ten  to  fifteen  tubules,  situated  between  the  two 
layers  of  the  broad  ligament  and  in  close  proximity  to  the 
ovary;  these  constitute  what  is  known  as  the  epoophoron 
{parovarium  or  organ  of  Rosenmiiller) .  The  tubules  ter- 
minate blindly  at  the  ends  nearest  the  ovary,  but  at  the  other 
extremity,  where  they  are  somewhat  coiled,  they  open  into 
a  collecting  duct  which  represents  the  upper  end  of  the 
Wolffian  duct.  Near  this  rudimentary  body  is  another, 
also  composed  of  tubules,  representing  the  remains  of  the 


THE    GENITAL    DUCTS. 


379 


excretory  portion  of  the  mesonephros  and  termed  the  paro- 
ophoron. So  far  as  the  mesonephros  is  concerned,  there- 
fore, the  persisting  rudiments  in  the  female  are  comparable 
to  those  occurring  in  the  male. 

As  regards  the  ducts,  however,  the  case  is  different,  for 
in  the  female  it  is  the  Miillerian  ducts  which  persist,  while 
the  Wolffians  undergo  degeneration,  a  small  portion  of  their 

W£  Ep 

HM  "*^ 


i-rz> 


UM 


FEMALE 


INDIFFERENT 


MALE 


Fig.  211. — Diagrams  Illustrating  the  Transformation  of  the 
mullerian   and  wolffian   ducts. 

B,  Bladder;  C,  clitoris;  CG,  canal  of  Gaertner;  CI,  cloaca;  B,o,  epo- 
ophoron ;  Ep,  epididymis  ;  F ,  Fallopian  tube ;  G,  genital  gland ;  HE, 
hydatid  of  epididymis;  HM,  hydatid  of  Morgagni;  K,  kidney;  MD, 
Miillerian  duct ;  O,  ovary ;  P ,  penis ;  Po,  paroophron ;  Pr,  prostate 
gland;  R,  rectum;  T,  testis;  IJ ,  urethra;  UM,  uterus  masculinus ; 
Ur,  ureter;  JJS,  urogenital  sinus;  [/?,  uterus;  V,  vagina;  VA,  vas 
aberrans ;  VD,  vas  deferens ;  VS,  vesicula  seminalis ;  WB,  Wolffian 
body;   WD,  Wolffian  duct. —  {Modified  from  Huxley.) 


380  THE    GENITAL   DUCTS. 

upper  ends  persisting  in  connection  with  the  epoophora, 
while  tlieir  lower  ends  persist  as  straight  tubules  lying  at 
the  sides  of  the  vagina  and  forming  what  are  known  as  the 
canals  of  Gartner.  The  Miillerian  ducts,  on  the  other  hand, 
become  converted  into  the  Fallopian  tubes  {tubes  uterince), 
and  in  their  lower  portions  into  the  uterus  and  vagina. 
From  the  margins  of  the  openings  by  which  the  Miillerian 
ducts  communicate  with  the  coelom  projections  develop  at 
an  early  period  and  give  rise  to  the  HinhricE,  with  the  excep- 
tion of  the  one  connected  with  the  ovary,  the  fimbria  ovarica, 
which  is  the  upper  persisting  portion  of  the  original  genital 
ridge,  its  lower  portion,  below  the  ovary,  being  represented 
by  the  ovarian  and  inguinal  ligament  already  described.  It 
has  been  seen  that  the  lower  portions  of  the  Miillerian  ducts 
fuse  together  to  form  a  single  canal,  and  it  is  from  this  that 
the  uterus  and  vagina  are  differentiated,  the  histological 
distinction  of  the  two  portions  commencing  to  manifest 
itself  at  about  the  third  month.  During  the  fourth  month 
the  vaginal  portion  of  the  duct  becomes  flattened  and  the 
epithelium  lining  its  lumen  fuses  so  as  to  completely  occlude 
it  and,  a  little  later,  there  appears  near  its  lower  opening 
a  distinct  semicircular  fold  attached  to  its  dorsal  margin. 
This  is  the  hymen,  a  structure  which  seems  to  be  repre- 
sented in  the  male  by  the  veru  uiontanum.  The  oblitera- 
tion of  the  lumen  of  the  vagina  persists  until  about  the 
sixth  month,  when  the  cavity  is  re-established  by  the  break- 
ing down  of  the  central  epithelial  cells. 

The  diagram.  Fig.  211,  illustrates  the  transformation 
from  the  indifferent  condition  which  occurs  in  the  two  sexes, 
and  that  the  homologies  of  the  various  parts  may  be  clearly 
understood  they  may  also  be  stated  in  tabular  form  as 
on  the  next  page. 

In  addition  to  the  sessile  hydatid,  a  stalked  hydatid  also 
occurs  in  connection  with  the  testis,  and  a  similar  structure  is 


THE   BLADDER. 


381 


Indifferent  Stage. 

Male. 

Female. 

Genital  ridge -| 

Testis. 

Gubernaculum.                   -j 

Fimbria  ovarica. 
Ovary. 

Ovarian  ligament. 
Round  ligament. 

Wolfifian  body ^ 

Globus  major  of  epididymis. 

Paradidymis. 

Vasa  aberrantia. 

Epoophoron. 
Paroophoron. 

Wolffian  ducts < 

Body  and  globus  minor  of 

epididymis. 
Vasa  deferentia. 
Ejaculatory  ducts. 

Collecting  tubules  of   epo- 
ophoron. 

Canals  of  Gartner. 

Miillerian  ducts < 

Sessile  hydatid. 
Uterus  masculinus. 

Fallopian  tubes. 

Uterus. 

"Vagina. 

attached  to  the  fimbriated  opening  of  each  Fallopian  tube. 
The  significance  of  these  structures  is  uncertain,  though  it  has 
been  suggested  that  they  are  persisting  rudiments  of  the  prone- 
phros. 

A  failure  of  the  development  of  the  various  parts  just  de- 
scribed to  be  completed  in  the  normal  manner  leads  to  various 
abnormalities  in  connection  with  the  reproductive  organs. 
Thus  there  may  occur  a  failure  in  the  fusion  of  the  lower  por- 
tions of  the  Miillerian  ducts,  a  bihorned  or  bipartite  uterus 
resulting,  or  the  two  ducts  may  come  into  contact  and  their 
adjacent  walls  fail  to  disappear,  the  result  being  a  median  par- 
tition separating  the  vagina  or  both  the  vagina  and  uterus  into 
two  compartments.  The  excessive  development  of  the  fold 
which  gives  rise  to  the  hymen  may  lead  to  a  complete  closure 
of  the  lower  opening  of  the  vagina,  while,  on  the  other  hand, 
a  failure  of  the  Miillerian  ducts  to  fuse  may  produce  a  biper- 
forate  hymen. 

The  Development  of  the  Urinary  Bladder  and  the 
Urogenital  Sinus. — So  far  the  relations  of  the  lower  ends 
of  the  urinogenital  ducts  have  not  been  considered  in  detail, 
although  it  has  been  seen  that  in  the  early  stages  of  develop- 
ment the  Wolffian  and  Miillerian  ducts  open  into  the  sides 
of  the  ventral  portion  of  the  cloaca;  that  the  ureters  com- 
municate with  the  lower  portions  of  the  Wolffian  ducts; 
that  from  the  ventral  anterior  portion  of  the  cloaca  the 


382 


THE    BLADDER. 


allantoic  duct  extends  outward  into  the  belly-stalk;  and, 
finally  (p..  297),  that  the  cloaca  becomes  divided  into  a 
dorsal  portion,  which  forms  the  lower  part  of  the  rectum, 
and  a  ventral  portion,  which  is  continuous  with  the  allan- 
tois  and  receives  the  urinogenital  ducts  (Fig.  212).  It  is 
the  history  of  this  ventral  portion  of  the  cloaca  which  is 
now  to  be  considered. 

It  may  be  regarded  as  consisting  of  two  portions,  an 


Fig.   212. — Reconstruction   of   the   Cloacal  Region   of  an   Embryo 

OF   14   MM. 
al,   Allantois ;    b,  bladder ;    gt,   genital   tubercle ;    /,   intestine ;   n,   spinal 

cord ;  nc,  notochord ;  r,  rectum ;  sg,  urogenital  sinus ;  ur,  ureter ; 

w,  Wolffian  duct. —  (Keibel.) 

anterior  and  a  posterior,  the  line  of  insertion  of  the  urino- 
genital ducts  marking  the  junction  of  the  two.  The  ante- 
rior or  upper  portion  is  destined  to  give  rise  to  the  urinar)- 
bladder  (Fig.  212,  b),  while  the  lower  one  forms  what  is 
known  for  a  time  as  the  urogenital  sinus  (sg).  The  blad- 
der, when  first  differentiated,  is  a  tubular  structure,  whose 
lumen  is  continuous  with  that  of  the  allantois,  but  after  the 


THE    BLADDER. 


383 


second  month  it  enlarges  to  become  more  sac-like,  while 
the  intra-embryonic  portion  of  the  allantois  degenerates  to 
a  solid  cord  extending  from  the  apex  of  the  bladder  to  the 
umbilicus  and  is  known  as  the  urachus.  During  the  en- 
largement of  the  bladder  the  terminal  portions  of  the  urino- 
genital  ducts  are  taken  up  into  its  walls,  a  process  which 
continues  until  finally  the  ureters  and  Wolffian  ducts  open 


Fig.  213.— Reconstruction  of  the  Cloacal  Structures   of  an   Em- 
bryo OF  25   MM. 
bl.  Bladder;  m,  Miillerian  duct;  r,  rectum;  sg,  urogenital  sinus;   sy, 

symphysis    pubis;    u,    ureter;    ur,    urethra;    w,    Wolffian    duet. — 

{Adapted  from  Keibel.) 

into  it  separately,  the  ureters  opening  to  the  sides  of  and  a 
little  anterior  to  the  ducts.  This  condition  is  reached  in 
embryos  of  about  14  mm.  (Fig.  212),  and  in  later  stages 
the  interval  between  the  two  pairs  of  ducts  is  increased 
(Fig.  213),  resulting  in  the  formation  of  a  short  canal 
connecting  the  lower  end  of  the  bladder  which  receives  the 
ureters  with  the  upper  end  of  the  urogenital  sinus,   into 


384  THE   UROGENITAL    SINUS. 

which  the  Wolffian  and  Miillerian  ducts  open.  This  con- 
necting canal  represents  the  urethra  (Fig.  213,  ur),  or 
rather  the  entire  urethra  of  the  female  and  the  proximal 
part  of  that  of  the  male,  since  a  considerable  portion  of 
the  latter  canal  is  still  undeveloped  (see  p.  386),  From 
this  urethra  there  is  developed,  at  about  the  third  month, 
a  series  of  solid  longitudinal  folds  which  project  upon  the 
outer  surface  and  separate  from  the  urethra  from  above 
downward.  These  represent  the  tubules  of  the  prostate 
gland  and  are  developed  in  both  sexes,  although  they  re- 
main in  a  somewhat  rudimentary  condition  in  the  female. 
The  muscular  tissue,  so  characteristic  of  the  gland  in  the 
adult  male,  is  developed  from  the  surrounding  mesenchyme 
at  a  later  stage. 

The  urogenital  sinus  is  in  the  early  stages  also  tubular 
in  its  upper  part,  though  it  expands  considerably  below, 
where  it  is  closed  by  the  cloacal  membrane.  This,  by  the 
separation  of  the  cloaca  into  rectum  and  sinus,  has  become 
divided  into  two  portions,  the  more  ventral  of  which  closes 
the  sinus  and  the  dorsal  the  rectum,  the  interval  between 
them  having  become  considerably  thickened  to  form  the 
perineal  body.  In  embryos  of  about  17  mm.  the  urogenital 
portion  of  the  membrane  has  broken  through,  and  in  later 
stages  the  tubular  portion  of  the  sinus  is  gradually  taken 
up  into  the  more  expanded  lower  portion,  until  finally  the 
entire  sinus  forms  a  shallow  depression,  termed  the  vesti- 
bule, into  the  upper  part  of  which  the  urethra  opens,  while 
below  are  the  openings  of  the  Wolffian  (ejaculatory)  ducts 
in  the  male  or  the  orifice  of  the  vagina  in  the  female. 
From  the  sides  of  the  lower  part  of  the  sinus  a  pair  of 
evaginations  arise  toward  the  end  of  the  fourth  month  and 
give  rise  to  the  bidbo-vcstibular  glands  {Bartholin's)  of 
the  female  or  the  corresponding  bulbo-urethral  glands 
(Cozuper's)  in  the  male. 


THE    EXTERNAL    GENITALIA. 


38S 


The  Development  of  the  External  Genitalia. — At  about 
the  fifth  week,  before  the  urogenital  sinus  has  opened  to 
the  exterior,  the  mesenchyme  on  its  ventral  wall  begins  to 
thicken,  producing  a  slight  projection  to  the  exterior.  This 
eminence,  which  is  known  as  the  genital  tubercle  (Fig.  212, 
gt),  rapidly  increases  in  size,  its  extremity  becomes  some- 
what bulbously  enlarged  (Fig.  214,  gl)  and  a  groove,  ex- 
tending to  the  base  of  the  terminal  enlargement,  appears 
upon  its  vestibular  surface,  the  lips  of  the  groove  forming 


/-y^ 


^y 


^(/S 


Fig.   214. — The  External  Genitalia  of  an   Embryo  of  25   mm. 

a,  Anus;  gf,  genital  fold;  gl,  glans;  gs,  genital  swelling;  p,  perineal 

body. —  (  Keibel. ) 

two  well-marked  genital  folds  (Fig.  214,  gf).  At  about  the 
tenth  week  there  appears  on  either  side  of  the  tubercle  an 
enlargement  termed  the  genital  szvelling  (Fig.  214,  gs), 
which  is  due  to  a  thickening  of  the  mesenchyme  of  the 
lower  part  of  the  ventral  abdominal  wall  in  the  region 
where  the  inguinal  ligament  is  attached,  and  with  the  ap- 
pearance of  these  structures  the  indifferent  stage  of  the 
external  genitals  is  completed. 

In  the  female  the  growth  of  the  genital  tubercle  proceeds 

rather  slowly  and  it  becomes  transformed  into  the  clitoris, 

the  genital   folds  becoming  prolonged  to   form   tlie   labia 

luinora.     The  genital  swellings  increase  in  size,  their  mes- 

34 


386  THE    EXTERNAL    GENITALIA. 

enchyme  becomes  transformed  into  a  mass  of  adipose  and 
fibrous  tissue  and  they  become  converted  into  the  labia 
majora,  the  interval  between  them  constituting-  the  vulva. 

In  the  male  the  early  stages  of  development  are  closely 
similar  to  those  of  the  female;  indeed,  it  has  been  well  said 
that  the  external  genitals  of  the  adult  female  resemble  those 
of  the  fetal  male.  In  early  stages  the  genital  tubercle  elon- 
gates to  form  the  penis  and  the  integument  which  covers  the 
proximal  part  of  it  grows  forward  as  a  fold  which  encloses 
the  bulbous  enlargement  or  glans  and  forms  the  prepuce, 
whose  epithelium  fuses  with  that  covering  the  glans  and 
only  separates  from  it  later  by  a  cornification  of  the  cells 
along  the  plane  of  fusion.  The  genital  folds  meet  together 
and  fuse,  converting  the  vestibule  and  the  groove  upon  the 
vestibular  surface  of  the  penis  into  the  terminal  portion  of 
the  male  urethra  and  bringing  it  about  that  the  vasa  defer- 
entia  and  the  uterus  masculinus  open  upon  the  floor  of  that 
passage.  The  two  genital  swellings  are  at  the  same  time 
brought  closer  together,  so  as  to  lie  between  the  base  of 
the  penis  and  the  perineal  body  and,  eventually,  they  form 
the  scrotum.  The  mesenchyme  of  which  they  were  pri- 
marily composed  differentiates  into  the  same  layers  as  are 
found  in  the  wall  of  the  abdomen  and  a  peritoneal  pouch 
is  prolonged  into  them  from  the  abdomen,  so  that  they  form 
sacs  into  which  the  testes  descend  toward  the  close  of  fetal 
Hfe  (p.  388). 

The  homologies  of  the  portions  of  the  reproductive  appa- 
ratus derived  from  the  cloaca  and  of  the  external  genitalia 
in  the  two  sexes  may  be  perceived  from  the  following  table. 

Numerous  anomalies,  depending  upon  an  inhibition  or  ex- 
cess of  the  development  of  the  parts,  may  occur  in  connection 
with  the  external  genitalia.  Should,  for  instance,  the  lips  of 
the  groove  on  the  vestibular  surface  of  the  penis  fail  to  fuse, 
the  penial  portion  of  the  urethra  remains  incomplete,  consti- 
tuting a  condition  known  as  hypospadias,  a  condition  which 


THE    DESCENT    OF    THE    OVARIES. 


387 


Male. 

Female. 

Urogenital  sinus 

Urinary  bladder. 

Proximal  portion  of  urethra. 

Bulbo-urethral  glands. 

The  rest  of  the  urethra. 

Penis. 

Prepuce  and  integument  of 

penis. 
Scrotum. 

Urinary  bladder. 
Urethra. 

Bulbo-vestibular  glands. 
Vestibule. 

Genital  tubercle 

Clitoris. 

Genital  folds 

Labia  minora. 

Genital  swellings  .., 

Labia  majora. 

It  is  stated  above  that  the  layers  which  compose  the  walls  of 
the  scrotum  are  identical  with  those  of  the  abdominal  wall. 
This  may  be  seen  in  detail  from  the  following  scheme : 


Abdominai,  Walls. 
Integument. 
Superficial  fascia. 
External  oblique  muscle. 
Internal  oblique  muscle. 
Transverse  muscle. 
Peritoneum. 


Scrotum. 

Integument. 
Dartos. 

Intercolumnar    fascia. 
Cremasteric  fascia. 
Infundibuliform  fascia. 
Tunica  vaginalis. 


offers  a  serious  bar  to  the  fulfilment  of  the  sexual  act.  If  the 
hypospadias  is  complete  and  there  be  at  the  same  time  an 
imperfect  development  of  the  penis,  as  frequently  occurs  in 
such  cases,  the  male  genitalia  closely  resemble  those  of  the 
female  and  a  condition  is  produced  which  is  usually  known  as 
hermaphroditism.  It  is  noteworthy  that  in  such  cases  there 
is  frequently  a  somewhat  excessive  development  of  the  uterus 
masculinus,  and  a  similar  condition  may  be  produced  in  the 
female  by  an  excessive  development  of  the  clitoris.  Such 
cases,  however,  which  concern  only  the  accessory  organs  of 
reproduction,  are  instances  of  what  is  more  properly  termed 
spurious  hcrinaphroditism,  true  hermaphroditism  being  a  term 
which  should  be  reserved  for  possible  cases  in  which  the  geni- 
tal ridges  give  rise  in  the  same  individual  to  both  ova  and 
spermatozoa.  Such  cases  are  of  exceeding  rarity  in  the  human 
species,  although  occasionally  observed  in  the  lower  vertebrates, 
and  the  great  majority  of  the  examples  of  hermaphroditism 
hitherto  observed  are  cases  of  the  spurious  variety. 

The  Descent  of  the  Ovaries  and  Testes. — The  posi- 
tions finally  occupied  by  the  ovaries  and  testes  are  very  dif- 
ferent from  those  which  they  possess  in  the  earlier  stages 
of  development,  and  this  is  especially  true  in  the  case  of  the 


388  THE    DESCENT    OF    THE    TESTES, 

testes.  The  change  of  position  is  partly  due  to  the  rate  of 
growth  of  the  inguinal  ligaments  being  less  than  that  of 
the  abdominal  walls,  the  reproductive  organs  being  thereby- 
drawn  downward  toward  the  inguinal  regions  where  the 
ligaments  are  attached.  The  attachment  is  to  the  bottom 
of  a  slight  pouch  of  peritoneum  which  projects  a  short  dis- 
tance into  the  substance  of  the  genital  swellings  and  is 
known  as  the  canal  of  Nuck  in  the  female,  and  in  the  male 
as  the  vaginal  process. 

In  the  female  a  second  factor  combines  with  that  just 
mentioned.  The  relative  shortening  of  the  inguinal  liga- 
ments acting  alone  would  draw  the  ovaries  toward  the 
inguinal  regions,  but  since  the  inguinal  ligaments  are  united 
to  the  Miillerian  ducts  (see  p.  372),  and  since  the  ovaries  are 
continuous  with  the  posterior  layer  of  the  peritoneal  folds 
which  contain  these  ducts,  the  fusion  of  the  lower  ends  of 
the  ducts  produces  a  traction  toward  the  median  line,  so  that 
the  ovaries  come  to  lie  finally  in  the  true  pelvis. 

With  the  testes  the  case  is  more  complicated,  since  in  addi- 
tion to  the  relative  shortening  of  the  inguinal  ligaments 
there  is  an  elongation  of  the  vaginal  processes  into  the  sub- 
stance of  the  genital  swellings,  and  it  must  be  remembered 
that  the  testes,  like  the  ovaries,  are  primarily  connected  with 
the  peritoneum.  Three  stages  may  be  recognized  in  the 
descent  of  the  testes.  The  first  of  these  depends  on  the 
slow  rate  of  elongation  of  the  inguinal  ligaments  or  guber- 
naculum.  It  lasts  until  about  the  fifth  month  of  develop- 
ment, when  the  testes  lie  in  the  inguinal  region  of  the 
abdomen,  but  during  this  month  the  elongation  of  the  guber- 
naculum  becomes  more  rapid  and  brings  about  the  second 
stage,  during  which  there  is  a  slight  ascent  of  the  testes,  so 
that  they  come  to  lie  a  little  higher  in  the  abdomen.  This 
stage  is,  however,  of  short  duration,  and  is  succeeded  by 
tlie  stage  of  the  final  descent,  which  is  characterized  l)y  the 


THE    DESCENT    OF    THE    TESTES. 


389 


elongation  of  the  vaginal  processes  of  the  peritoneum  into 
the  substance  of  the  scrotum  (Fig.  215,  A).  Since  the 
gubernaculum  is  attached  to  the  bottom  of  the  process,  and 
since  its  growth  has  again  diminished,  the  testes  gradually 
assume  again  their  inguinal  position,  and  are  finally  drawn 
down  into  the  scrotum  with  the  vaginal  processes. 

The  condition  which  is  thus  acquired  persists  for  some 
time  after  birth,  the  testicles  being  readily  pushed  upward 
into  the  abdominal  cavit}^  along  the  cavity  by  which  they 


Fig.  215. — Diagrams  Illustrating  the  Descent  of  the  Testis. 
il.  Inguinal  ligament ;  m,  muscular  layer ;  s,  skin  and  dartos  of  the  scro- 
tum;  t,  testis;   tv,  tunica  vaginalis;  vd,  yas  deferens;  vp,  vaginal 
process  of  peritoneum. —  (After  Hertzmg.) 


descended.  Later,  however,  the  size  of  the  opening's  of  the 
vaginal  processes  into  the  general  peritoneal  cavity  becomes 
greatly  reduced,  so  that  each  process  becomes  converted  into 
an  upper  narrow  neck  and  a  lower  sac-like  cavity  (Fig. 
215,  B),  and,  still  later,  the  walls  of  the  neck  portion  fuse 
and  become  converted  into  a  solid  cord,  while  the  lower 
portion,  wrapping  itself  around  the  testis,  becomes  the  tunica 
vaginalis  (tv).  By  these  changes  the  testes  become  per- 
manently located  in  the  scrotum.  During  the  descent  of 
the  testes  the  remains  of  each  Wolffian  body,  the  epididymis, 


390  THE    DESCENT    OF    THE    TESTES. 

and  the  upper  part  of  each  vas  deferens,  together  with  the 
spermatic  vessels  and  nerves;  are  drawn  down  into  the 
scrotum,  and  the  mesenterial  fold  in  which  they  were  origi- 
nally contained  and  which  is  comparable  to  the  broad  liga- 
ment of  the  female,  also  practically  disappears,  becoming 
converted  into  a  sheath  of  connective  tissue  which  encloses 
the  vas  deferens  and  the  vessels  and  nerves,  binding  them 
together  into  what  is  termed  the  spermatic  cord.  The  mes- 
orchium,  which  united  the  testis  to  the  peritoneum  enclos- 
ing the  Wolffian  body,  does  not  share  in  the  degeneration 
of  the  latter,  but  persists  as  a  fold  extending  between  the 
epididymis  and  the  testis  and  forming  the  sinus  epididymis. 

In  the  text-books  of  anatomy  tha  spermatic  cord  is  usually 
described  as  lying  in  an  inguinal  canal  which  traverses  the 
abdominal  walls  obliquely  immediately  above  Poupart's  liga- 
ment. So  long  as  the  lumen  of  the  neck  portion  of  the  vaginal 
process  of  peritoneum  remains  patent  there  is  such  a  canal, 
placing  the  cavity  of  the  tunica  vaginalis  in  communication 
with  the  general  peritoneal  cavity,  but  the  cord  does  not  trav- 
erse this  canal,  but  lies  outside  it  in  the  retroperitoneal  con- 
nective tissue.  When,  however,  the  neck  of  the  vaginal  proc- 
ess disappears,  a  canal  no  longer  exists,  although  the  connective 
tissue  which  surrounds  the  spermatic  cord  and  unites  it  with 
the  tissues  of  the  abdominal  walls  is  less  dense  than  the  neigh- 
boring tissues,  so  that  the  cord  may  readily  be  separated  from 
these  and  thus  appear  to  lie  in  a  canal. 

LITERATURE. 

/      B.  M.  Allen  :  "  The  Embryonic  Development  of  the  Ovary  and  Testes 

in  Mammals,"  Anicr.  Journ.  of  Anat.,  in,  1904. 
\/     W.  Felix  :  "  Entwickelungsgeschichte  des  Exkretions-systems,"  Ergebn. 

dcr  Anat.  unci  Entzvicklungsgcsch.,  xiii,   1903. 
A.  Fleischmann:  "  Morphologische  Studien  iiber  Kloake  und  Phallus 

der  Amnioten,"  Morphol.  Jahrhuch,  xxx,  xxxii  und  xxxvi,  1902- 

1904,  1907. 
O.  Frankl:  "  Beitrage  zur  Lehre  vom  Descensus  testiculorum,"  Sits- 

ungsbcr.  dcr  kais.  Akad.  Wissensch.  Wien,  Math.-N aturzviss.  Classe, 

cix,  1900. 
\/   S.  P.  Gage  :  "  A  Three  Weeks  Human  Embryo,  with  especial  reference 

to  the  Brain  and   the   Ncphric   System,"  Amer.   Journ.   of  Anat., 

IV,  1905. 


LITERATURE.  39  ^ 

'^     G.   C.   HuBER :    "  On  the  Development  and   Shape  of  the  Uriniferous 

Tubules   of   Certain   of  the    Higher   Mammals,"  Amer.   Journ.   of 

Anal,  TV,  Suppl.  1905. 
J.    Janosik  :    "  Histologisch-embryologische   Untersuchungen    uber   das 

Urogenitalsystem,"  Sitsungsbcr.  dcr  kais.  Akad.   Wissensch.   Wien, 

Math.-Naturzviss.  Classe,  xci,  1887. 
\^  F.  Keibel  :  "  Zur  Entwickelungsgeschichte  des  menschlichen  Urogenital- 

apparatus,"  Archiv  fur  Anat.  und  Physiol.,  Anat.  Abth.,  1896. 
J.  B.  Macallum  :  "  Notes  on  the  Wolffian  Body  of  Higher  Mammals," 

Amer.  Journ.  of  Anat.,  i,  1902. 

E.  Martin  :  "  Ueber  die  Anlage  der  Urniere  beim  Kaninchen,"  Archiv 

filr  Anat.  und  Physiol,  Anat.  Abth.,  1888. 

H.  Meyer  :  "  Die  Entwickelung  der  Urnieren  beim  Menschen,"  Archiv 
filr  mikrosk.  Anat.,  xxxvi,  1890. 

G.  VON  Mihalkovicz  :  "  Untersuchungen  iiber  die  Entwickelung  des 
Harn-  und  Geschlechtsapparates  der  Amnioten,"  Internat.  Monats- 
schrift  fiir  Anat.  und  Physiol.,  n,  1885. 
]/  W.  Nagel  :  "  Ueber  die  Entwickelung  des  Urogenitalsystems  des  Men- 
schen," Archiv  fiir  niikros.  Anat.,  xxxiv^  1889. 
'/  W.  Nagel  :  "  Ueber  die  Entwickelung  des  Uterus  und  der  Vagina  beim 
Menschen,"  Archiv  fiir  mikrosk.  Anat.,  xxxvii^  1891. 

W.   Nagel  :   "  Ueber  die  Entwickelung  der  innere  und  aussere  Geni- 
talien  beim  menschlichen  Weibes,"  Archiv  filr  Gyndkol.,  xlv,  1894. 

G.    Pallin  :    "  Beitrage   zur   Anatomic   der   Prostata    und   der   Samen- 
blasen,"  Archiv  fiir  Anat.  und  Physiol.,  Anat.  Abth.,  1901. 

A.  Soulie:  "  Sur  la  migration  des  Testicules,"  Comptes  Rendus  de  la 
Soc.  de  Biol.  Paris,  Ser.  lome,  11,  1895. 
y      A.  Soulie  :  "  Sur  le  mecanisme  de  la  migration  des  testicules,"  Comp- 
tes Rendus  de  la  Soc.  de  Biol.  Paris,  Ser.  lome,  11,  .1895. 
V     J.  Tandler:  "Ueber  Vornieren-Rudimente  beim  menschliche  Embryo," 
Anat.  Hefte,  xxviii,  1905. 

F.  Tourneux  :    "  Sur    le    developpement    et    revolution    du    tubercule 

genital  chez  le  foetus  humain  dans  les  deux  sexes,"  Journ.  de 
I' Anat.  et  de  la  Physiol.,  xxv,  1889. 
S.  Weber  :  "  Zur  Entwickelungsgeschichte  des  uropoetischen  Apparates 
bei  Saugern,  mit  besonderer  Beriicksichtigung  der  Urniere  zur 
Zeit  des  Auftretens  der  bleibenden  Niere,"  Morphol.  Arbeiten,  \ii, 
1897. 


CHAPTER  XIV. 
THE  SUPRARENAL  SYSTEM  OF  ORGANS. 

To  the  suprarenal  system  a  number  of  bodies  of  peculiar 
structure,  probably  concerned  with  internal  secretion,  may 
be  assigned.  In  the  fishes  they  fall  into  two  distinct 
groups,  the  one  containing  organs  derived  from  the  coelomic 
epithelium  and  known  as  interrenal  organs,  and  the  other 
consisting  of  organs  derived  from  the  sympathetic  nervous 
,  system  and  which,  on  account  of  the  characteristic  affinity 
they  possess  for  chromium  salts,  have  been  termed  the 
chromaffine  organs.  But  in  the  amphibia  and  amniote 
vertebrates,  while  both  the  groups  are  represented  by  inde- 
pendent organs,  yet  they  also  become  intimately  associated 
to  form  the  suprarenal  glands,  so  that,  notwithstanding 
their  distinctly  different  origins,  it  is  convenient  to  consider 
them  together. 

The  Development  of  the  Suprarenal  Bodies. — The 
suprarenal  bodies  make  their  appearance  at  an  early  stage, 
while  the  Wolffian  bodies  are  still  in  a  well-developed  con- 
dition, and  they  are  situated  at  first  to  the  medial  side  of 
the  upper  ends  of  these  structures  (Fig.  203,  sr).  Their 
final  relation  to  the  metanephros  is  a  secondary  event,  and 
is  merely  a  topographic  relation,  there  being  no  develop- 
mental relation  between  the  two  structures. 

In  the  human  embryo  they  make  their  appearance  at 
about  the  beginning  of  the  fourth  week  of  development 
as  a  number  of  proliferations  of  the  coelomic  epithelium, 
which  project  into  the  subjacent  mesenchyme,  and  are  situ- 
ated on  either  side  of  the  median  line  between  the  root  of 

392 


DEVELOPMENT    OF    THE    SUPRARENAL    BODIES.  393 

the  mesentery  and  the  upper  portion  of  the  Wolffian  body. 
The  various  proHferations  soon  separate  from  the  epith'e- 
lium  and  unite  to  form  two  m.asses  situated  in  the  mesen- 
chyme one  on  either  side  of  the  upper  portion  of  the  abdom- 
inal aorta.  In  certain  forms,  such  as  the  rabbit,  the  pri- 
mary proliferations  arise  from  the  bottom  of  depressions 
of  the  coelomic  epithelium  (Fig.  216),  but  in  the  human 
embryo  these  depressions  do  not  form. 

Up  to  this  stage  the  structure  is  a  pure  interrenal  organ, 
but  during  the  fifth  week  of  development  masses  of  cells, 
derived   from   the  abdominal   portion   of   the   sympathetic 


I 


— n^d 


Ao 


•1'// 

i      'VSi 

i 

/  c  -  ''^:£J 

Fig.  216. — Section  through   a   Portion  of  the  Wolffian  Ridge  of 

A  Rabbit  Embryo  of  6.5  mm. 

Ao,  Aorta;  ns,  nephrostome ;  Sr,  suprarenal  body;  vc,  cardinal  vein; 

wc,  tubule  of  Wolffian  body;  wd,  Wolfifian  duct. —  (Aichcl.) 

nervous  system,  begin  to  penetrate  into  each  of  the  inter- 
renal masses  (Fig.  217),  and  form  strands  traversing  them. 
At  about  the  ninth  or  tenth  week  fatty  granules  begin  to 
appear  in  the  interrenal  cells  and  somewhat  later,  about  the 
fourth  month,  the  sympathetic  constituents  begin  to  show 
their  chromaffine  characteristics.  The  two  tissues,  how- 
ever, remain  intermingled  for  a  considerable  time,  and  it  is 
not  until  a  much  later  period  that  they  become-  definitely 
separated,  the  sympathetic  elements  gradually  concentrat- 
ing in  the  centre  of  the  compound  organ  to  become  its 
medullary  substance,  while  the  interrenal  tissue  forms  the 
cortical  substance.     Indeed,  it  is  not  until  after  birth  that 


394  DEVELOPMENT    OF    THE    SUPRARENAL    BODIES. 

the  separation  of  the  two  tissues  and  their  histological  dif- 
ferentiation is  complete,  occasional  masses  of  interrenal  tis- 
sue remaining  imheclded  in  the  medullary  substance  and  an 
immigration  of  sympathetic  cells  continuing  until  at  least 
the  tenth  year  (Wiesel). 

A  great  deal  of  difference  of  opinion  has  existed  in  the  past 
concerning  the  origin  of  the  suprarenal  glands.  By  several 
authors  they  have  been  regarded  as  derivatives  in  whole  or  in 
part  of  the  excretory  apparatus,  some  tracing  their  origin  to 
the  mesonephros  and  others  even  to  the  pronephros.     The  fact 


Fig.   217. — Section   through   the   Supr.\renal   Body  of  an   Embryo 

OF    17    MM. 

A,  Aorta  ;   R,  interrenal   portion  ;  S .  sympathetic  nervous  system ;  SB , 
sympathetic   cells  penetrating  the   interrenal   portion. —  (Wiesel.) 

that  in  some  mammals  tlie  cortical  (interrenal)  cells  are 
formed  from  the  bottom  of  depressions  of  the  ccKlomic  epi- 
thelium seemed  to  lend  support  to  this  view,  but  it  is  now 
pretty  firmly  established  that  the  appearances  thus  presented 
do  not  warrant  the  interpretation  placed  upon  them  and  that 
the  interrenal  tissue  is  derived  from  the  coelomic  epithelium 
quite  independently  of  the  nephric  tubules.  That  the  chroinaf- 
fine  tissue  is  a  derivative  of  the  sympathetic  nervous  system 
has  long  been  recognized. 

During  the  development  of  the  suprarenal  glands  por- 


DEVELOPMENT    OF    THE    SUPRARENAL    BODIES.  395 

tions  of  their  tissue  may  be  separated  as  the  result  of  un- 
equal growth  and  form  what  are  commonly  spoken  of  as 
accessory  suprarenal  glands,  although,  since  they  are  usu- 
ally composed  solely  of  cortical  substance,  the  term  acces- 
sory mterrenal  bodies  would  be  more  appropriate.  They 
may  be  formed  at  different  periods  of  development  and 
occur  in  various  situations,  as  for  instance,  in  the  vicinity 
of  the  kidneys  or  even  actually  imbedded  in  their  substance, 
on  the  walls  of  neighboring  blood-vessels,  in  the  retroperi- 
toneal tissue  below  the  level  of  the  kidneys,  and  in  connec- 
tion with  the  organs  of  reproduction,  in  the  spermatic  cord, 
epididymis  or  rete  testis  of  the  male  and  in  the  broad  liga- 
ment of  the  female. 

It  seems  probable  that  the  bodies  associated  with  the 
reproductive  apparatus  are  separated  from  the  main  mass 
of  interrenal  tissue  before  the  immigration  of  the  sympa- 
thetic tissue  and  before  the  descent  of  the  ovaries  or  testes, 
while  those  which  occur  at  higher  levels  are  of  later  origin, 
and  in  some  cases  may  contain  some  medullary  substance, 
being  then  true  accessory  suprarenals.  Such  bodies  are, 
however,  comparatively  rare,  the  great  majority  of  the 
accessory  bodies  being  composed  of  interrenal  tissue  alone. 

Independent  chromaffine  organs  also  occur,  among  them 
the  intercarotid  ganglia  and  the  organs  of  Zuckerkandl 
being  especially  deserving  of  note.  It  may  also  be  pointed 
out,  however,  that  the  chromaffine  cells  have  the  same  origin 
as  the  cells  of  the  sympathetic  ganglia  and  may  sometimes 
fail  to  separate  from  the  latter,  so  that  the  sympathetic 
ganglia  and  plexuses  frequently  contain  chromaffine  cells. 

The  Intercarotid  Ganglia. — These  structures,  which  are 
frequently  though  incorrectly  termed  carotid  glands,  are 
small  bodies  about  5  mm.  in  length,  which  lie  usually  to 
the  mesial  side  of  the  upper  ends  of  the  common  carotid 
arteries.     They   possess    a   very   rich   arterial   supply   and 


396 


THE    INTERCAROTID    GANGLIA. 


stand  in  intimate  relation  with  the  branches  of  an  inter- 
carotid  sympathetic  plexus,  and,  furthermore,  they  are 
characterized  by  possessing  as  their  specific  constituents 
markedly  chromaffine  cells,  among  which  are  scattered 
stellate  cells  resembling  the  cells  of  the  sympathetic  ganglia. 
They  have  been  found  to  arise  in  pig  embryos  of  44  mm. 
by  the  separation  of  cells  from  the  ganglionic  masses  scat- 
tered throughout  the  carotid  sympathetic  plexuses.     These 


Fig.  218. — Section  of  a  Cell  Ball  from  the  Intercarotid  Ganglion 

OF  Man. 

be,   Blood  capillaries ;   ev,  efferent  vein ;   S,   connective-tissue  septum ; 

I,  trabeculse. —  (From  Bohm  and  Davidoff,  after  S diaper.) 

cells,  which  become  the  chromaffine  cells,  arrange  them- 
selves in  round  masses  termed  cell  balls,  many  of  which 
unite  to  form  each  ganglion,  and  in  man  each  cell  ball 
becomes  broken  up  into  trabeculae  by  the  blood-vessels  (Fig. 
218)  which  penetrate  its  substance,  and  the  individual  balls 
are  separated  from  one  another  by  considerable  quantities 
of  connective  tissue. 


THE    INTERCAROTID    GANGLIA.  397 

Some  confusion  has  existed  in  the  past  as  to  the  origin  of 
this  structure.  The  mesial  wall  of  the  proximal  part  of  the 
internal  carotid  artery  becomes  considerably  thickened  during 
the  early  stages  of  development  and  the  thickening  is  traversed 
by  numerous  blood  lacunae  which  communicate  with  the  lumen 
of  the  vessel.  This  condition  is  perhaps  a  relic  of  the  branchial 
capillaries  which  in  the  lower  gill-breathing  vertebrates  repre- 
sent the  proximal  portion  of  the  internal  carotid,  and  has  noth- 
ing to  do  with  the  formation  of  the  intercarotid  ganglion, 
although  it  has  been  believed  by  some  authors  (Schaper)  that 
the  ganglion  was  derived  from  the  thickening  of  the  wall  of 
the  vessel.  The  fact  that  in  some  animals,  such  as  the  rat  and 
the  dog,  the  ganglion  stands  in  relation  with  the  external  caro- 
tid and  receives  its  blood-supply  from  that  vessel  is  of  import- 
ance in  this  connection. 

The  thickening  of  the  internal  carotid  disappears  in  the 
higher  vertebrates  almost  entirely,  but  in  the  Amphibia  it  per- 
sists throughout  life,  the  lumen  of  the  proximal  part  of  the 
vessel  being  converted  into  a  fine  meshwork  by  the  numerous 
trabeculse  which  traverse  it.  This  carotid  labyrinth  has  been 
termed  the  carotid  gland,  a  circumstance  which  has  probably 
assisted  in  producing  confusion  as  to  the  real  significance  of 
the  intercarotid  ganglion. 

The  Organs  of  Ziickerkandl. — In  embryos  of  14.5  mm. 
there  have  been  found,  in  front  of  the  abdominal  aorta, 
closely  packed  groups  of  cells  which  resemble  in  appear- 
ance the  cells  composing  the  ganglionated  cord,  two  of  these 
groups,  which  extend  downward  along  the  side  of  the  aorta 
to  belov^  the  point  of  origin  of  the  inferior  mesenteric  artery, 
being  especially  distinct.  These  cell  groups  give  rise  to  the 
ganglia  of  the  prgevertebral  sympathetic  plexuses  and  also 
to  peculiar  bodies  which,  from  their  discoverer,  may  be 
termed  the  organs  of  Zuckerkandl.  Each  body  stands  in 
intimate  relation  with  the  fibers  of  the  sympathetic  plexuses 
and  has  a  rich  blood-supply,  resembling  in  these  respects 
the  intercarotid  ganglia,  and  the  resemblance  is  further 
increased  by  the  fact  that  the  specific  cells  of  the  organ  are 
markedly  chromafiine. 

At  birth  the  bodies  situated  in  the  upper  portion  of  the 


398 


THE    ORGANS    OF    ZUCKERKANDL. 


abdominal  cavity  have  broken  up  into  small  masses,  but 
the  two  lower  ones,  mentioned  above,  are  still  well  defined 
(Fig.  219).  Even  these,  however,  seem  to  disappear  later 
on  and  no  traces  of  them  have  as  yet  been  found  in  the 
adult. 


■bl.a 


Fig.  219. — Organs  of  Zuckerkandl  from  a   New-born  Child. 
a,  Aorta;  ci,  inferior  vena  cava;  i.c,  common  iliac  artery;  mi,  inferior 
mesenteric   artery ;    n.l   and   n.r,   left   and   right   accessory   organs ; 
pl.a,  aortic  plexus;  u,  ureter;  v.r.s,  left  renal  vein. —  {Zuckerkandl.') 

LITERATURE. 

A.  KoHN :   "  Ueber  den  Ban  und  die  Entwickelung  der  sog.   Carotis- 

driise,"  Archtv.  fiir  mikrosk.  Anat.,  lvi,  1900. 
A.   KoHN :   "  Das  chromaffine   Gevvebe,"  Ergcbn.   dcr  Anat.   und  Ent- 

wickelungsgesch.,  xii^  1902. 


LITERATURE.  399 

H.  Poll  :   "  Die  vergleichende  Entwickelungsgeschichte  der  Nebeunieren- 

systeme  der  Wirbeltiere,"  Hertwig's  Handb.  der  vergl.  tind  exper. 

Entzi'icklungslehre  der  Wirbeltiere,  iii,   igo6. 
A.  SouLiE :  "  Recherches  sur  le  developpement  des  capsules  surrenales 

chez  les  Vertebres,"  Jown.  de  I'Anat.  et  de  la  Physiol.,  xxxix,  1903. 
J.   WiESEL :    "'  Beitrage   znr   Anatomic   und   Entwickelung   der   mensch- 

lichen  Nebenniere,"  Anat.  Heft.,  xix,  1902. 
E.   ZucKERKANDL :    "  Ucbcr   Nebenorgane  des    Sympathicus   im  Retro- 

peritonealraum    des    Menschen,"    Verhandl.    Anat.    Gesellsch.,    xv, 

1901. 


CHAPTER  XV. 

THE   DEVELOPMENT   OF  THE   NERVOUS 
SYSTEM. 

The  Histogenesis  of  the  Nervous  System. — The  entire 
central  nervous  system  is  derived  from  tlie  cells  lining  the 
medullary  groove,  whose  formation  and  conversion  into  the 
medullary  canal  has  already  been  described  (p.  98).  When 
the  groove  is  first  formed,  the  cells  lining  it  are  some- 
what more  columnar  in  shape  than  those  on  either  side  of 
it,  though  like  them  they  are  arranged  in  a  single  layer; 
later  they  increase  by  mitotic  division  and  arrange  them- 
selves in  several  layers,  so  that  the  ectoderm  of  the  groove 
becomes  very  much  thicker  than  that  of  the  general  surface 
of  the  body.  While  its  tissue  is  in  this  condition  the  lips 
of  the  groove  unite,  and  the  subsequent  differentiation  of 
the  canal  so  formed  differs  somewhat  in  different  regions, 
although  a  fundamental  plan  may  be  recognized.  This 
plan  is  most  readily  perceived  in  the  region  which  becomes 
the  spinal  cord,  and  may  be  described  as  seen  in  that  region. 

Throughout  the  earlier  stages,  the  cells  lining  the  inner 
wall  of  the  medullary  tube  are  found  in  active  proliferation, 
some  of  the  cells  so  produced  arranging  themselves  with 
their  long  axes  at  right  angles  to  the  central  canal  and 
extending  throughout  the  entire  thickness  of  the  wall  to 
form  a  supportive  framework  (Fig.  220),  while  others, 
whose  destiny  is  for  the  most  part  not  yet  determinable, 
and  which  therefore  may  be  termed  indiifcrcnt  cells,  occur 
in  the  meshes  of  this  framework.  At  this  stage  a  trans- 
verse section  of  the  medullary  tube  shows  it  to  be  composed 

400 


THE    HISTOGENESIS    OF    THE    NERVOUS    SYSTEM. 


401 


of  two  well-defined  zones,  an  inner  one  immediately  sur- 
rounding the  central  canal  and  composed  of  the  indifferent 
cells  and  the  bodies  of  the  supportive  or  ependymal  cells, 
and  an  outer  one  consisting  of  the  branched  prolongations 
of  the  ependymal  cells.  This  outer  layer  is  termed  the 
marginal  velum  (Randschleier)  (Fig.  220,  mv).  The  in- 
different cells  now  be- 
gin to  wander  outward 
to  form  a  definite 
layer,  termed  the  man- 
tle layer,  lying  between 
the  marginal  velum 
and  the  bodies  of  the 
ependymal  cells  ( Fig. 
221),  and  when  this 
layer  has  become  well 
established  the  cells 
composing  it  begin  to 
divide  and  to  differen- 
tiate into  ( I )  cells 
termed  neuroblasts , 
destined  to  become 
nerve-cells,  and  (2) 
others  which  appear  to 
be  supportive  in  charac- 
ter and  are  termed  neu- 
roglia cells  (Fig.  221, 
B).  The  latter  are  for 
the  most  part  small 
and  have  their  cell-bodies  drawn  out  into  very  numerous 
and  exceedingly  slender  processes,  which  ramify  among 
the  neuroblasts,  these,  on  the  other  hand,  being  larger  and 
each  early  developing  a  single  strong  process  which  grows 
out  into  the  marginal  velum  and  is  known  as  an  axis- 
35 


Fig.  220. — Ependymal  Cells  from  the 
Spinal     Cord     of     an     Embryo     of 

4.25    MM. 

mv,  Marginal  velum. —  (His.) 


402       THE    HISTOGENESIS   OF   THE   NERVOUS   SYSTEM, 

cylinder.  At  a  later  period  the  neuroblasts  also  give  rise 
to  other  processes,  termed  dendrites,  more  slender  and 
shorter  than  the  axis-cylinders,  branching  repeatedly  and, 
as  a  rule,  not  extending  beyond  the  limits  of  the  mantle 
layer. 

The  axis-cylinder  processes  of  the  majority  of  the  neuro- 
blasts on  reaching  the  marginal  velum  bend  upward  or 
downward  and,  after  traversing  a  greater  or  less  length 
of  the  cord,  re-enter  the  mantle  layer  and  terminate  by 


OoO  ^«*/ 

or- 


o.4r 


.^.-o^^«T4 


Fig.    221. — Diagrams    showing    the    Development   of    the    Mantle 

Layer  in  the   Spinal  Cord. 
The  circles,  indifferent  cells ;  circles  with  dots,  neuroglia  cells ;  shaded 

cells,  germinal  cells;  circles  with  cross,  germinal  cells  in  mitosis; 

black  cells,  nerve-cells.-^  (5"c/!a^^r.) 

dividing  into  numerous  short  branches  which  come  into 
relation  with  the  dendrites  of  adjacent  neuroblasts.  The 
processes  of  certain  cells  situated  in  the  ventral  region  of 
the  mantle  zone  pass,  however,  directly  through  the  mar- 
ginal velum  out  into  the  surrounding  tissues  and  constitute 
the  ventral  nerve-roots  (Fig.  224). 

The  dorsal  nerve-roots  have  a  very  different  origin.     In 
embryos  of  about  2.5  mm.,  in  ^ which  the  medullary  canal 


THE    HISTOGENESIS    OF    THE    NERVOUS    SYSTEM.        4O3 


is  only  partly  closed  (Fig.  42),  the  cells  which  lie  along  the 
line  of  transition  between  the  lips  of  the  groove  and  the 
general  ectoderm  form  a  distinct  ridge  readily  recognized  in 
sections  and  termed  the  neural  crest  (Fig.  222,  A).  When 
the  lips  of  the  groove  fuse  together  the  cells  of  the  crest 
unite  to  form  a  wedge-shaped  mass,  completing  the  closure 
of  the  canal  (Fig.  222,  B),  and  later  proliferate  so  as  to 
extend  outward  over  the 
surface  of  the  canal 
(Fig.  222,  C).  Since  this 
proliferation  is  most  ac- 
tive in  the  regions  of  the 
crest  which  correspond 
to  the  mesodermic  somites 
there  is  formed  a  series 
of  cell  masses,  arranged 
segmentally  and  situated 
in  the  mesenchyme  at  the 
sides  of  the  medullary 
canal  (Fig.  206).  These 
cell-masses  represent  the 
dorsal  root  ganglia,  and 
certain  of  their  constitu- 
ent cells,  which  may  also 
be  termed  neuroblasts, 
early  assume  a  fusiform 
shape  and  send  out  a  process  from  each  extremity.  One 
of  these  processes,  the  axis-cylinder,  grows  inward  toward 
the  medullary  canal  and  penetrates  its  marginal  velum,  and, 
after  a  longer  or  shorter  course  in  this  zone,  enters  'the 
mantle  layer^and  comes  into  contact  with  the  dendrites  of 
some  of  the  central  neuroblasts.  The  other  process  extends 
peripherally  and  is  to  be  regarded  as  an  extremely  elongated 
dendrite.     The  processes  from  the  cells  of  each  ganglion 


Fig.  222. — Three  Sections  through 
THE  Medullary  Canal  of  an  Em- 
bryo OF  2.5  MM. — {von  Lenhossek.) 


404       THE    HISTOGENESIS    OF    THE    NERVOUS    SYSTEM. 

aggregate  to  form  a  nerve,  that  formed  by  the  axis-cyhnders 
being  the  posterior  root  of  a  spinal  nerve,  while  that  formed 
by  the  dendrites  soon  unites  with  the  ventral  nerve-root  of 
the  corresponding  segment  to  form  the  main  stem  of  a 
spinal  nerve. 

There  is  thus  a  very  important  difference  in  the  mode 
of  development  of  the  two  nerve-roots,  the  axis-cylinders 
of  the  ventral  roots  arising  from  cells  situated  in  the  wall 
of  the  medullary  canal  and  growing  outward  (centrifu- 
gally),  while  those  of  the  dorsal  root  spring  from  cells  situ- 


FiG.    223. — Cells    from    the   Gasserian    Ganglion   of   a    Guinea-pig 

Embryo. 

a,   Bipolar  cell ;    b   and    c,   transitional   stages   to   d,   T-shaped   cells. — 

(voii  Gelnichtcn.) 

ated  peripherally  and  grow  inward  (centripetally)  toward 
the  medullary  canal.  In  the  majority  of  the  dorsal  root 
ganglia  the  points  of  origin  of  the  two  processes  of  each 
bi -polar  cell  gradually  approach  one  another  and  eventually 
come  to  rise  from  a  common  stem,  a  process  of  the  cell-body, 
which  thus  assumes  a  characteristic  T  form. 

From  what  has  been  said  it  will  be  seen  that  each  axis- 
cylinder  is  an  outgrowth  from  a  single  neuroblast  and  is  part 
of  its  cell-body,  as  are  also  the  dendrites.  Another  view  has, 
however,  been  advanced  to  the  ciTect  that  the  nerve  fibers  first 


THE    HISTOGENESIS    OF    THE    NERVOUS    SYSTEM.        405 

appear  as  chains  of  cells  and  that  the  axis-cylinders,  being 
differentiated  from  the  cytoplasm  of  the  chains,  are  really 
multicellular  products.  Many  difficulties  stand  in  the  way  of 
the  acceptance  of  this  view  and  recent  observations,  both  his- 
togenetic  (Cajal)  and  experimental  (Harrison)  tend  to  con- 
firm the  unicellular  origin  of  the  axis-cylinders.  The  embryo- 
logical  evidence  therefore  goes  to  support  the  neurone  theory, 
which  regards  the  entire  nervous  system  as  composed  of  sepa- 
rate units,  each  of  which  corresponds  to  a  single  cell  and  is 
termed  a  neurone. 

By  the  development  of  the  axis-cylinders  which  occupy 
the  meshes  of  the  marginal  velum,  that  zone  increases  in 
thickness  and  comes  to  consist  principally  of  nerve-fibers, 
while  the  cell-bodies  of  the  neurones  of  the  cord  are  situ- 
ated in  the  mantle  zone.  No  such  definite  distinction  of 
color  in  the  two  zones  as  exists  in  the  adult  is,  however, 
noticeable  until  a  late  period  of  development,  the  medullary 
sheaths,  which  give  to  the  nerve-fibers  their  white  appear- 
ance not  beginning  to  appear  until  the  fifth  month  and  con- 
tinuing to  form  from  that  time  onward  until  after  birth. 
The  origin  of  the  myelin  which  composes  the  medullary 
sheaths  is  as  yet  uncertain,  although  the  more  recent  obser- 
vations tend  to  show  that  it  is  picked  out  from  the  blood 
and  deposited  around  the  axis-cylinders  in  some  manner  not 
yet  understood.  Its  appearance  is  of  importance  as  being 
associated  with  the  beginning  of  the  functional  activity  of 
the  nerve-fibers. 

In  addition  to  the  medullary  sheaths  the  majority  of  the 
fibers  of  the  peripheral  nervous  system  are  provided  with 
primitive  sheaths,  which  are  lacking,  however,  to  the  fibers 
of  the  central  system.  They  are  formed  by  cells  which 
wander  out  from  the  dorsal  root-ganglia  and  are  therefore 
of  ectodermal  origin.  Frog  larvae  deprived  of  their  neural 
crests  at  an  early  stage  of  development  produce  ventral  nerve 
fibers  altogether  destitute  of  primitive  sheaths  (Harrison). 


406        THE    HISTOGENESIS    OF    THE   NERVOUS    SYSTEM. 

Various  theories  have  been  advanced  to  account  for  the 
formation  of  the  medullary  sheaths.  It  has  been  held  that  the 
myelin  is  formed  at  the  expense  of  the  outermost  portions  of 
the  axis-cylinders  themselves  (von  Kolliker),  and,  on  the  other 
hand,  it  has  been  regarded  as  an  excretion  of  the  cells  which 
compose  the  primitive  sheaths  surrounding  the  fibers  (Ran- 
vier),  a  theory  which  is,  however,  invalidated  by  the  fact  that 
myelin  is  formed  around  the  fibers  of  the  central  nervous  sys- 
tem which  possess  no  primitive  sheaths.  As  stated  above,  the 
more  recent  observations  (Wlassak)  indicate  its  exogenous 
origin. 

It  has  been  seen  that  the  central  canal  is  closed  in  the 
mid-dorsal  line  by  a  mass  of  cells  derived  from  the  neural 
crest.  These  cells  do  not  take  part  in  the  formation  of 
the  mantle  layer,  but  become  completely  converted  into 
ependymal  tissue,  and  the  same  is  true  of  the  cells  situated 
in  the  mid-ventral  line  of  the  canal.  In  these  two  regions, 
known  as  the  roof-plate  and  floor-plate  respectively,  the 
wall  of  the  canal  has  a  characteristic  structure  and  does  not 
share  to  any  great  extent  in  the  increase  of  thickness  which 
distinguishes  the  other  regions  (Fig.  224),  In  the  lateral 
walls  of  the  canal  there  is  also  noticeable  a  differentiation 
into  two  regions,  a  dorsal  one  standing  in  relation  to  the 
ingrowing  fibers  from  the  dorsal  root  ganglia  and  known 
as  the  dorsal  zone,  and  a  ventral  one,  the  ventral  zone,  simi- 
larly related  to  the  ventral  nerve-roots.  In  different  regions 
of  the  medullary  tube  these  zones,  as  well  as  the  roof-  and 
floor-plates,  undergo  different  degrees  of  development,  pro- 
ducing peculiarities  which  may  now  be  considered. 

The  Development  of  the  Spinal  Cord. — Even  before 
the  lips  of  the  medullary  groove  have  met  a  marked  enlarge- 
ment of  the  anterior  portion  of  the  canal  is  noticeable,  the 
region  which  will  become  the  brain  being  thus  distinguished 
from  the  more  posterior  portion  which  will  be  converted 
into  the  spinal  cord.  When  the  formation  of  the  meso- 
(lermic  somites  is  completed,  the  spinal  cord  terminates  at 


THE    SPINAL    CORD,  4O7 

the  level  of  the  last  somite,  and  in  this  region  still  retains 
its  connection  with  the  ectoderm  of  the  dorsal  surface  of 
the  body;  but  in  that  portion  of  the  cord  which  is  posterior 
to  the  first  coccygeal  segment  the  histological  differentiation 
does  not  proceed  beyond  the  stage  when  the  walls  consist 
of  several  layers  of  similar  cells,  the  formation  of  neuro- 
blasts and  nerve-roots  ceasing  with  the  segment  named. 
After  the  fourth  month  the  more  differentiated  portion 
elongates  at  a  much  slower  rate  than  the  surrounding  tissues 
and  so  appears  to  recede  up  the  spinal  canal,  until  its  ter- 
mination is  opposite  the  second  lumbar  vertebra.  The  less 
differentiated  portion,  which  retains  its  connection  with  the 
ectoderm  until  about  the  fifth  month,  is,  on  the  other  hand, 
drawn  out  into  a  slender  filament  whose  cells  degenerate 
during  the  sixth  month,  except  in  its  uppermost  part,  so  that 
it  comes  to  be  represented  throug"hout  the  greater  part  of 
its  extent  by  a  thin  cord  composed  of  pia  mater.  This  cord 
is  the  structure  known  in  the  adult  as  the  filum  terminale, 
and  lies  in  the  center  of  a  leash  of  nerves  occupying  the 
lower  part  of  the  spinal  canal  and  termed  the  cauda  eqidna. 
The  existence  of  the  cauda  is  due  to  the  recession  of  the 
cord  which  necessitates  for  the  lower  lumbar,  sacral  and 
coccygeal  nerves,  a  descent  through  the  spinal  canal  for  a 
greater  or  less  distance,  before  they  can  reach  the  inter- 
vertebral foramina  through  which  they  make  their  exit. 

In  the  early  stages  of  development  the  central  canal  of 
the  cord  is  cjuite  large  and  of  an  elongated  oval  form,  but 
later  it  becomes  somewhat  rhomboidal  in  shape  (Fig.  224, 
A),  the  lateral  angles  marking  the  boundaries  between  the 
dorsal  and  ventral  zones.  As  development  proceeds  the 
sides  of  the  canal  in  the  dorsal  region  gradually  approach 
one  another  and  eventually  fuse,  so  that  this  portion  of 
the  canal  becomes  obliterated  (Fig.  224,  B)  and  is  indi- 
cated by  the  dorsal  longitudinal  fissure  in  the  adult  cord, 


4o8 


THE    SPINAL    CORD. 


the  central  canal  of  which  corresponds  to  the  ventral  por- 
tion only  of  the  embryonic  cavity.  While  this  process  has 
been  going  on  both  the  roof-  and  the  floor-plate  have  be- 
come depressed  below  the  level  of  the  general  surface  of 
the  cord,  and  by  a  continuance  of  the  depression  of  the  floor- 
plate — a  process  really  due  to  the  enlargement  and  conse- 
quent bulging  of  the  ventral  zone — the  anterior  median  fis- 


FiG.  224. — Transverse  Sections  through  the  Spinal  Cords  of  Em- 
bryos OF  (A)  about  Four  and  a  Half  Weeks  and  (B)  about 
Three  Months. 

cB,  Fasciculus  of  Burdach;  cG,  fasciculus  of  Goll ;  dh,  dorsal  horn;  ds, 
dorsal  zone ;  fp,  floor-plate ;  ob,  oval  bundle ;  rp,  roof-plate ;  vh, 
ventral  horn;  va,  ventral  zone. —  (His.) 

sure  is  produced,  the  difference  between  its  shape  and  that 
of  the  dorsal  fissure  being  due  to  the  difference  in  its  devel- 
opment. 

The  development  of  the  mantle  layer  proceeds  at  first 
more  rapidly  in  the  ventral  zone  than  in  the  dorsal,  so  that 
at  an  early  stage  (Fig.  224,  A)  the  anterior  horn  of  gray 


THE    BRAIN.  4O9 

matter  is  much  more  pronounced,  but  on  the  development 
of  the  dorsal  nerve-roots  the  formation  of  neuroblasts  in 
the  dorsal  zone  proceeds  apace,  resulting  in  the  formation 
of  a  dorsal  horn.  A  small  portion  of  the  zone,  situated 
between  the  point  of  entrance  of  the  dorsal  nerve-roots  and 
the  roof-plate,  fails,  however,  to  give  rise  to  neuroblasts  and 
is  entirely  converted  into  ependyma.  This  represents  the 
future  fasciculus  of  Goll  (Fig.  224,  A,  cG),  and  at  the  point 
of  entrance  of  the  dorsal  roots  into  the  cord  a  well-marked 
oval  bundle  of  fibers  is  formed  (Fig.  224,  A,  oh)  which,  as 
development  proceeds,  creeps  dorsally  over  the  surface  of 
the  dorsal  horn  until  it  meets  the  lateral  surface  of  the  fas- 
ciculus of  Goll,  and,  its  further  progress  toward  the  median 
line  being  thus  impeded,  it  insinuates  itself  between  that 
fasciculus  and  the  posterior  horn  to  form  the  fascicuhts  of 
Burdach  (Fig.  224,  B,  cB). 

Little  definite  is  as  yet  known  concerning  the  development 
of  the  other  fasciculi  which  are  recognizable  in  the  adult  cord, 
but  it  seems  certain  that  the  lateral  and  anterior  cerebro-spinal 
(pyramidal)  fasciculi  are  composed  of  fibers  which  grow 
downward  in  the  meshes  of  the  marginal  velum  from  neuro- 
blasts situated  in  the  cerebral  cortex,  while  the  cerebello-spinal 
(direct  cerebellar)  fasciculi  and  the  fibers  of  the  ground- 
bundles  have  their  origin  from  cells  of  the  mantle  layer  of 
the  cord. 

The  myelination  of  the  fibers  of  the  spinal  cord  begins 
between  the  fifth  and  sixth  months  and  appears  first  in  the 
fasciculi  of  Burdach,  and  about  a  month  later  in  the  fasciculi 
of  Goll.  The  myelination  of  the  great  motor  paths,  the  lateral 
and  anterior  cerebro-spinal  fasciculi,  is  the  last  to  develop, 
appearing  toward  the  end  of  the  ninth  month  of  fetal  life. 

The  Development  of  the  Brain. — The  enlargement  of 
the  anterior  portion  of  the  medullary  canal  does  not  take 
place  quite  uniformly,  but  is  less  along  two  transverse  lines 
than  elsewhere,  so  that  the  brain  region  early  becomes  di- 
vided into  three  primary  vesicles  which  undergo  further  dif- 
ferentiation as  follows.  Upon  each  side  of  the  anterior 
36 


4IO 


THE    BRAIN. 


-Jny 


vesicle  an  evagination  appears  and  becomes  converted  into  a 
club-shaped  structure  attached  to  the  ventral  portion  of  the 
vesicle  by  a  pedicle.  These  evaginations  (Fig.  225,  op)  are 
known  as  the  optic  evaginations,  and  being  concerned  in  the 
formation  of  the  eye  will  be  considered  in  the  succeeding 

chapter.  After  their  forma- 
tion the  antero-lateral  portions 
of  the  vesicle  become  bulged 
out  into  two  protuberances  {h) 
which  rapidly  increase  in  size 
and  give  rise  eventually  to  the 
two  cerebral  . hemispheres, 
which  form,  together  with  the 
portion  of  the  vesicle  which  lies 
between  them,  what  is  termed 
the  telencephalon  or  fore-brain, 
the  remainder  of  the  vesicle 
giving  rise  to  what  is  known 
as  the  diencephalon  or  'tzveen- 
brain  (Fig.  225,  /).  The  mid- 
dle vesicle  is  bodily  converted 
into  the  mesencephalon  or  mid- 
hrain  {ni) ,  but  the  posterior 
vesicle  differentiates  so  that 
three  parts  may  be  recognized : 
(i)  a  rather  narrow  portion 
which  immediately  succeeds 
the  mid-brain  and  is  termed  the 
isthmus  (i)  ;  (2)  a  portion 
whose  roof  and  floor  give  rise 
to  the  cerebellum  and  pons  respectively,  and  which  is  termed 
the  metencephalon  or  hind-brain  (mt)  ;  and  (3)  a  terminal 
portion  which  is  known  as  the  medulla  oblongata,  or,  to 
retain   a   consistent   nomenclature,   the   myelencephalon  or 


Fig.  225. — Reconstruction  of 
THE  Brain  of  an  Embryo  of 

2.15    MM. 

h,  Hemisphere ;  i,  isthmus ;  m, 
mesencephalon ;  w/,  mid-brain 
flexure;  mt,  metencephalon; 
my,  myelencephalon  ;  nf,  neck 
flexure;  ot,  otic  capsule;  op, 
optic  evagination ;  t,  dien- 
cephalon.—  (His.) 


THE    BRAIN. 


411 


after-brain  (my).  From  each  of  these  six  divisions  de- 
finite structures  arise  whose  relations  to  the  secondary 
divisions  and  to  the  primary  vesicles  may  be  understood 
from  the  following  table  and  from  the  annexed  figure  ( Fig, 
226),  which  represents  a  median  longitudinal  section  of 
the  brain  of  a  fetus  of  three  months. 

'  Myelencephalon        Medulla  oblongata  (I). 

Pons  (II  i). 


3rd   Vesicle, < 


r  Pons   (II  i). 
Metencephalon    |  Cerebellum    (II  2). 


Isthmus 


Brachia  conjunctiva   (III). 
Cerebral    peduncles    (posterior 
portion). 


r  Cerebral     peduncles     (anterior 

2nd  Vesicle, Mesencephalon-^       portion)     (IV  i). 

(^  Corpora  quadrigemina   (IV  2). 


I  St  Vesicle, ■< 


Diencephalon 


Telencephalon 


Pars  mammillaris 
Thalamus  (V  2). 
Epiphysis   (V  3). 


(V  I). 


Infundibulum    (VI    i). 
Corpus  striatum   (VI  2). 
Olfactory  bulb   (VI  3). 
Hemispheres  (VI  4). 


But  while  the  walls  of  the  primary  vesicles  undergo  this 
complex  differentiation,  their  cavities  retain  much  more  per- 
fectly their  original  relations,  only  that  of  the  first  vesicle 
sharing  to  any  great  extent  the  modifications  of  the  walls. 
The  cavity  of  the  third  vesicle  persists  in  the  adult  as  the 
fourth  ventricle,  traversing  all  the  subdivisions  of  the  vesi- 
cle; that  of  the  second,  increasing  but  little  in  height  and 
breadth,  constitutes  the  aqucudncttis  cerebri;  while  that  of 
the  first  vesicle  is  continued  into  the  cerebral  hemispheres  to 
form  the  lateral  ventricles,  the  remainder  of  it  constituting 


412 


THE    BRAIN. 


the  tliivd  ventricle,  which  indudes  the  cavity  of  the  median 
portion  of  the  telencephalon  as  well  as  the  entire  cavity  of 
the  diencephalon. 

During  the  differentiation  of  the  various  divisions  of  the 
brain  certain  flexures  appear  in  the  roof  and  floor,  and  to  a 
certain  extent  correspond  with  those  already  described  as 
occurring  in  the  embryo.  The  first  of  these  flexures  to 
appear  occurs  in  the  region  of  the  mid-brain,  the  first  vesicle 


Fig.  226.- 


-Median  Longitudinal  Section  of  the  Brain  of  an  Embryo 
OF  THE  Third  Month. —  (His.) 


being  bent  ventrally  until  it  comes  to  lie  at  practically  a 
right  angle  with  the  axis  of  the  mid-brain.  This  may  be 
termed  the  mid-brain  flexure  (Fig.  225,  mf)  and  corre- 
sponds with  the  head-bend  of  the  embryo.  The  second 
flexure  occurs  in  the  region  of  the  medulla  oblongata  and 
is  known  as  the  neck  flexure  (Fig.  225,  nf)  ;  it  corresponds 
with  the  similarly  named  bend  of  the  embryo  and  is  pro- 
duced by  a  bending  ventrally  of  the  entire  head,  so  that  the 
axis  of  the  mid-brain  comes  to  lie  almost  at  right  angles 
with  that  of  the  medulla  and  that  of  the  first  vesicle  parallel 


THE    MYELENCEPHALON,  413 

with  it.  Finally,  a  third  flexure  occurs  in  the  region  of  the 
metencephalon  and  is  entirely  peculiar  to  the  nervous-  sys- 
tem; it  consists  of  a  bending  ventrally  of  the  floor  of  the 
hind-brain,  the  roof  of  this  portion  of  the  brain  not  being 
affected  by  it,  and  it  may  consequently  be  known  as  the 
pons  flexure. 

In  the  later  development  the  pons  flexure  practically  dis- 
appears, owing  to  the  development  in  this  region  of  the 
transverse  fibers  and  nuclei  of  the  pons,  but  the  mid-brain 
and  neck  flexures  persist,  though  greatly  reduced  in  acute- 
ness,  the  axis  of  the  anterior  portion  of  the  adult  brain 
being  inclined  to  that  of  the  medulla  at  an  angle  of  about 
134  degrees. 

The  Development  of  the  Myelencephalon. — In  its  poste- 
rior portion  the  myelencephalon  closely  resembles  the  spinal 
cord  and  has  a  very  similar  development.  More  anteriorly, 
however,  the  roof-plate  (Fig.  227,  rp)  widens  to  form  an 
exceedingly  thin  membrane,  the  posterior  velum;  with  the 
broadening  of  the  roof-plate  there  is  associated  a  broaden- 
ing of  the  dorsal  portion  of  the  brain  cavity,  the  dorsal  and 
ventral  zones  bending  outward,  until,  in  the  anterior  por- 
tion of  the  after-brain,  the  margins  of  the  dorsal  zone  have 
a  lateral  position,  and  are,  indeed,  bent  ventrally  to  form 
a  reflected  lip  (Fig.  227).  The  portion  of  the  fourth 
ventricle  contained  in  this  division  of  the  brain  becomes 
thus  converted  into  a  broad  shallow  cavity,  whose  floor  is 
formed  by  the  ventral  zones  separated  in  the  median  line 
by  a  deep  groove,  the  floor  of  which  is  the  somewhat  thick- 
ened floor-plate.  About  the  fourth  month  there  appears  in 
the  roof-plate  a  transverse  groove  into  which  the  surround- 
ing mesenchyme  dips,  and,  as  the  groove  deepens  in  later 
stages,  the  mesenchyme  contained  within  it  becomes  con- 
verted into  blood-vessels,  forming  the  chorioid  plexus  of  the 
fourth  ventricle,  a  structure  which,  as  may  be  seen  from  its 


414 


THE    MYELENCEPHALON. 


development,  does  not  lie  within  the  cavity  of  the  ventricle, 
but  is  separated  from  it  by  the  portion  of  the  roof-plate 
which  forms  the  floor  of  the  groove. 

In  embryos  of  about  9  mm,  the  differentiation  of  the 
dorsal  and  ventral  zones  into  ependymal  and  mantle  layers 
is  clearly  visible  (Fig.  227),  and  in  the  ventral  zone  the 
marginal  velum  is  also  well  developed.  Where  the  fibers 
from  the  sensory  ganglion  of  the  vagus  nerve  enter  the 
dorsal  zone  an  oval  area  (Fig.  227,  fs)  is  to  be  seen  which 
is  evidently  comparable  to  the  oval  bundle  of  the  cord  and 


Fig.    227. — Transverse   Section   through    the    Medulla   Oblongata 

OF  an  Embryo  of  9.1  mm. 

ds,  Dorsal  zone ;  fp,  floor-plate ;  fs,  fasciculus  solitarius ;  /,  lip ;  rp,  roof- 

plate;  vs,  ventral  zone;  X  and  XII,  tenth  and  twelfth  nerves. —  (His.) 


consequently  with  the  fasciculus  of  Burdach.  It  gives  rise 
to  the  solitary  fasciculus  of  adult  anatomy,  and  in  embryos 
of  II  to  13  mm.  it  becomes  covered  in  by  the  fusion  of  the 
reflected  lip  of  the  dorsal  zone  with  the  sides  of  the  myeleii- 
cephalon,  this  fusion,  at  the  same  time,  drawing  the  mar- 
gins of  the  roof-plate  ventrally  to  form  a  secondary  lip 
(Fig.  228).  Soon  after  this  a  remarkable  migration  ven- 
trally of  neuroblasts  of  the  dorsal  zone  begins.     Increasing 


THE    MYELENCEPHALON. 


415 


rapidly  in  number  the  migrating-  cells  pass  on  either  side 
of  the  solitary  fasciculus  toward  the  territory  of  the  ventral 
zone,  and,  passing  ventrally  to  the  ventral  portion  of  the 
mantle  layer,  into  which  fibers  have  penetrated  and  which 
becomes  the  formatio  reticularis  (Fig.  228,  fr),  they  dif- 
ferentiate to  form  the  olivary  body  (ol). 

The  thickening  of  the  floor-plate  gives  opportunity  for 
fibers  to  pass  across  the  median  line  from  one  side  to  the 
other,  and  this  opportunity  is  taken  advantage  of  at  an 
early  stage  by  the  axis-cylinders  of  the  neuroblasts  of  the 


Fig.  228. — Transverse  Section  through  the  Medulla  Oblongata 
OF  AN  Embryo  of  about  Eight  Weeks. 

av.  Ascending  root  of  the  trigeminus ;  fr,  reticular  formation ;  ol,  olivary- 
body;  sf,  solitary  fasciculus;  tr,  restiform  body;  XII,  hypoglossal 
nerve. —  (His.) 

ventral  zone,  and  later,  on  the  establishment  of  the  olivary 
bodies,  other  fibers,  descending  from  the  cerebellum,  de- 
cussate in  this  region  to  pass  to  the  olivary  body  of  the 
opposite  side.  In  the  lower  part  of  the  medulla  fib.ers  from 
the  neuroblasts  of  the  nuclei  of  Goll  and  Burdach,  which 
seem  to  be  developments  from  the  mantle  layer  of  the  dor- 
sal zone,  also  decussate  in  the  substance  of  the  floor-plate; 
these  fibers,  known  as  the  arcuate  fibers,  pass  in  part  to  the 
cerebellum,    associating   themselves    with    fibers    ascending 


4l6  THE    MYELENCEPHALON. 

from  the  spinal  cord  and  with  the  olivary  fibers  to  form  a 
round  bundle  situated  in  the  dorsal  portion  of  the  marginal 
velum  and  known  as  the  restiform  body  (Fig.  228,  tr). 

The  principal  differentiations  of  the  zones  of  the  myelen- 
cephalon  may  be  stated  in  tabular  form  as  follows: 

Roof-plate,    Posterior  velum 


Dorsal    zones,    < 


'  Nuclei  of  termination  of  sensory  roots 
of  cranial  nerves. 
Nuclei  of  Goll  and  Burdach. 
^  The  olivary  bodies. 


r  Nuclei  of  origin  of  the  motor  roots  of 

Ventral    zones,    J       cranial  nerves. 

[^  The  reticular  formation. 

Pjoor-plate,   The  median  raphe. 

The  Development  of  the  Metencephalon  and  Isthmus. — 
Our  knowledge  of  the  development  of  the  metencephalon, 
isthmus,  and  mesencephalon  is  by  no  means  as  complete 
as  is  that  of  the  myelencephalon.  The  pons  develops  as  a 
thickening  of  the  portion  of  the  brain  floor  which  forms 
the  anterior  wall  of  the  pons  flexure,  and  its  transverse 
fibers  are  well  developed  by  the  fourth  month  (Mihalko- 
vicz),  but  all  details  regarding  the  origin  of  the  pons  nuclei 
are  as  yet  wanting.  If  one  may  argue  from  what  occurs 
in  the  myelencephalon,  it  seems  probable  that  the  reticular 
formation  of  the  metencephalon  is  derived  from  the  ventral 
zone,  and  that  the  median  raphe  represents  the  floor-plate. 
Furthermore,  the  relations  of  the  pons  nuclei  to  the  reticu- 
lar formation  on  the  one  hand,  and  its  connection  by  means 
of  the  transverse  pons  fibers  with  the  cerebellum  on  the 
other,  suggest  the  possibility  that  they  may  be  the  meten- 
cephalic  representatives  of  the  olivary  bodies  and  are 
formed  by  a  migration  ventrally  of  neuroblasts  from  the 
dorsal  zones. 


THE    CEREBELLUM. 


417 


The  cerebellum  is  formed  from  the  dorsal  zones  and  roof- 
plate  of  the  metencephalon  and  is  a  thickening  of  the  tissue 
immediately  anterior  to  the  front  edge  of  the  posterior 
velum.  This  latter  structure  has  in  early  stages  a  rhom- 
boidal  shape  (Fig.  229,  A)  which  causes  the  cerebellar 
thickening  to  appear  at  first  as  if  composed  of  two  lateral 
portions  inclined  obliquely  toward  one  another.  In  reality, 
however,  the  thickening  extends  entirely  across  the  roof  of 
the  brain  (Fig.  229,  B),  the  roof-plate  probably  being 
invaded  by  cells  from  the  dorsal  zones  and  so  giving  rise 
to  the  vermis,  while  the  lobes  are  formed  directly  from  the 
dorsal  zones.     During  the  second  month  a  groove  appears 


Fig.  229. — A,  Dorsal  View  of  the  Brain  of  a  Rabbit  Embryo  of  16 

MM.;  B,  Median  Longitudinal  Section  of  a  Calf  Embryo  of  3  cm. 

c,    Cerebellum;    m,    mid-brain. —  (Mihalkovics.) 

on  the  ventral  surface  of  each  lobe,  marking  out  an  area 
which  becomes  the  flocculus,  and  later,  during  the  third 
month,  transverse  furrows  appear  upon  the  vermis  dividing 
it  into  five  lobes,  and  later  still  extend  out  upon  the  lobes 
and  increase  in  number  to  produce  the  lamellate  structure 
characteristic  of  the  cerebellum. 

The  histogenetic  development  of  the  cerebellum  at  first 
proceeds  along  the  lines  which  have  already  been  described 
as  typical,  but  after  the  development  of  the  mantle  layer 
the  cells  lining  the  greater  portion  of  the  cavity  of  the 
ventricle  cease  to  multiply,  only  those  which  are  situated 


4i8 


THE    CEREBELLUM. 


in  the  roof-plate  of  the  metencephalon  and  along  the  line 
of  junction  of  the  cerebellar  thickening  with  the  roof-plate 
continuing  to  divide.  The  indifferent  cells  formed  in  these 
regions  migrate  outward  from  the  median  line  and  forward 
in  the  marginal  velum  to  form  a  superficial  layer,  known 
as  the  epithelioid  layer,  and  cover  the  entire  surface  of  the 
cerebellum.  The  cells  of  this  layer,  like  those  of  the  man- 
tle, differentiate  into  neuroglia  cells  and  neuroblasts,  the 

latter  for  the  most  part 
migrating  centrally  at 
a  later  stage  to  mingle 
with  the  cells  of  the 
mantle  layer  and  to 
become  transformed  in- 
to the  granular  cells  of 
the  cerebellar  cortex. 
The  neuroglia  cells  re- 
main at  the  surface, 
however,  forming-  the 
principal  constituent  of 
the  outer  or,  as  it 
is  now  termed,  the  mo- 
lecular layer  of  the 
cortex,  and  into  this 
the    dendrites    of    the 


Fig.  230. — Diagram  Representing  the 
Differentiation  of  the  Cerebellar 
Cells. 

The  circles,  indifferent  cells ;  circles  with 
dots,  neuroglia  cells ;  shaded  cells,  ger- 
minal cells ;  circles  with  cross,  ger- 
minal cells  in  mitosis ;  black  cells, 
nerve-cells.  L,  Lateral  recess ;  M, 
median  furrow,  and  R,  floor  of  IVj 
fourth  ventricle. — (Schaper.) 


Purkinje  cells,  prob- 
ably derived  from  the  mantle  layer,  project.  The  migra- 
tion of  the  neuroblasts  of  the  epithelial  layer  is  probably 
completed  before  birth,  at  which  time  but  few  remain  in 
the  molecular  layer  to  form  the  stellate  cells  of  the  adult. 
The  origin  of  the  dentate  and  other  nuclei  of  the  cerebellum 
is  at  present  unknown,  but  it  seems  probable  that  they  arise 
from  cells  of  the  mantle  layer. 

The  nerve-fibers  which  form  the  medullary  substance  of 


THE    ISTHMUS.  419 

the  cerebellum  do  not  make  their  appearance  until  about  the 
sixth  month,  when  they  are  to  be  found  in  the  ependymal 
tissue  on  the  inner  surface  of  the  layer  of  granular  cells. 
Those  which  are  not  commissural  or  associative,  in  function 
converge  to  the  line  of  junction  of  the  cerebellum  with 
the  pons,  and  there  pass  into  the  marginal  velum  of  the 
pons,  myelencephalon,  or  isthmus  as  the  case  may  be. 

The  dorsal  surface  of  the  isthmus  is  at  first  barely  dis- 
tinguishable from  the  cerebellum,  but  as  development  pro- 
ceeds its  roof-plate  undergoes  changes  similar  to  those 
occurring  in  the  medulla  oblongata  and  becomes  converted 
into  the  anterior  veluin.  In  the  dorsal  portion  of  its  mar- 
ginal velum  fibers  passing  to  and  from  the  cerebellum  ap- 
pear and  form  the  hrachia  conjimctiva,  while  ventrally 
fibers,  descending  from  the  more  anterior  portions  of  the 
brain,  form  the  cerebral  peduncles.  Nothing  is  at  present 
known  as  to  the  history  of  the  gray  matter  of  this  division 
of  the  brain,  although  it  may  be  presumed  that  its  ventral 
zones  take  part  in  the  formation  of  the  tegmentum,  while 
from  its  dorsal  zones  the  nuclei  of  the  brachia  conjunctiva 
are  possibly  derived. 

The  following  table  gives  the  origin  of  the  principal  struc- 
tures of  the  metencephalon  and  isthmus : 

r  Posterior  velum.  Anterior  velum. 

1  Vermis    of   cerebellum. 

Lobes  of  cerebellum.  Brachia    conjunctiva. 

Flocculi. 

Nuclei     of     termination    of 

sensory    roots    of   cranial 

nerves. 
Pons  nuclei. 

Nuclei   of  origin   of  motor  Posterior  part  of  cere- 
Ventral  zones         -''       ™°^^    °^    cranial    nerves.       bral  peduncles. 

'  ' " '  1   Reticular  formation.  Posterior  part  of  teg- 
L  mentum. 

Floor-plate,  Median  raphe.  Median  raphe. 


Roof-plate, 


Dorsal   zones. 


420  THE    MESENCEPHALON. 

TJie  Development  of  flie  Meseneephaloii. — Our  knowl- 
edge of  the  development  of  this  portion  of  the  brain  is  again 
very  imperfect.  During  the  stages  when  the  flexures  of 
the  brain  are  well  marked  (Figs.  225  and  226)  it  forms  a 
very  prominent  structure  and  possesses  for  a  time  a  capa- 
cious cavity.  Later,  however,  it  increases  in  size  less  rap- 
idly than  adjacent  parts  and  its  walls  thicken,  the  roof-  and 
floor-plates  as  well  as  the  zones,  and,  as  a  result,  the  cavity 
becomes  the  relatively  smaller  canal-like  cerebral  aquseduct. 
In  the  marginal  velum  of  its  ventral  zone  fibers  appear  at 
about  the  third  month,  forming  the  anterior  portion  of  the 
cerebral  peduncles,  and,  at  the  same  time,  a  median  longi- 
tudinal furrow  appears  upon  the  dorsal  surface,  dividing  it 
into  two  lateral  elevations  which,  in  the  fifth  month,  are 
divided  transversely  by  a  second  furrow  and  are  thus  con- 
verted from  corpora  bigemina  (in  which  form  they  are 
found  in  the  lower  vertebrates)  into  corpora  qiiadrigemina. 

Nothing  is  known  as  to  the  differentiation  of  the  gray  mat- 
ter of  the  dorsal  and  ventral  zones  of  the  mid-brain.  From 
the  relation  of  the  parts  in  the  adult  it  seems  probable  that  in 
addition  to  the  nuclei  of  origin  of  the  oculomotor  and  trochlear 
nerves,  the  ventral  zones  give  origin  to  the  gray  matter  of  the 
tegmentum,  which  is  the  forward  continuation  of  the  reticular 
formation.  Similarly  it  may  be  supposed  that  the  corpora 
quadrigemina  are  developments  of  the  dorsal  zones,  as  may 
also  be  the  red  nuclei,  whose  relations  to  the  brachia  conjunc- 
tiva suggest  a  comparison  with  the  olivary  bodies  and  the 
nuclei  of  the  pons. 

A  tentative  scheme  representing  the  origin  of  the  mid-brain 
structures  may  be  stated  thus : 

Roof-plate,  (?) 


Dorsal  zones,   <  „ 


Corpora  quadrigemina. 


ed  nuclei. 

Nuclei   of  origin   of  the  third   and 

^-  ,  I       fourth  nerves. 

Ventral  zones, ->    .         .  ^      .  ^ 

'   Anterior  part  01  tegmentum. 

Anterior  part  of  cerebral  peduncles. 
Floor-plate,    Median  raphe. 


THE    DIENCEPHALON. 


421 


The  Developinent  of  the  Diencephalon. — A  transverse 
section  through  the  diencephalon  of  an  embryo  of  about  five 
weeks  (Fig.  231)  shows  clearly  the  differentiation  of  this 
portion  of  the  brain  into  the  typical  zones,  the  roof-plate 
{rp)  being  represented  by  a  thin-walled,  somewhat  folded 
area,  the  floor-plate  {fp)  by  the  tissue  forming  the  floor  of 
a  well-marked  ventral  groove,  while  each  lateral  wall  is 
divided  into  a  dorsal  and  ventral  zone  by  a  groove  known  as 
the  sulcus  Monroi  (Sin) ,  which 
extends  forward  and  ventrally 
toward  the  point  of  origin  of 
the  optic  evagination  (Fig. 
233).  At  the  posterior  end 
of  the  ridge-like  elevation 
which  represents  the  roof -plate 
is  a  rounded  elevation  (Fig. 
232,  p)  wdiich,  in  later  stages, 
elongates  until  it  almost  reaches 
the  dermis,  forming-  a  hollow 
evagination  of  the  brain  roof 
known  as  the  pineal  process. 
The  distal  extremity  of  this 
process  enlarges  to  a  sac-like 
structure  which  later  becomes 
lobed,  and,  by  an  active  pro- 
liferation of  the  cells  lining  the  cavities  of  the  various  lobes, 
finally  becomes  a  solid  structure,  the  pineal  body.  The 
more  proximal  portion  of  the  evagination,  remaining  hol- 
low, forms  the  pineal  stalk,  and  the  entire  structure,  body 
and  stalk,  constitutes  what  is  known  as  the  epiphysis. 

The  significance  of  this  organ  in  the  Mammalia  is  doubtful. 
In  the  Reptilia  and  other  lower  forms  the  outgrowth  is  double, 
a  secondary  outgrowth  arising  from  the  base  or  from  the 
anterior  wall  of  the  primary  one.  This  anterior  evagination 
elongates  until  it  reaches  the  dorsal  epidermis  of  the  head,  and, 


Fig.  231. — Transverse  Sec- 
tion OF  THE  Diencephalon 
OF  AN  Embryo  of  FrvE 
Weeks. 

ds,  Dorsal  zone ;  fpj  floor-plate ; 
rp,  roof-plate;  Sin,  sulcus 
Monroi ;  vs,  ventral  zone.- — • 
(His.) 


422 


THE    DIENCEPHALON. 


there  expanding,  develops  into  an  unpaired  eye,  the  epidermis 
which  overhes  it  becoming  converted  into  a  transparent  cor- 
nea.    In  the  JMammalia  this  anterior  process  does  not  develop 

and    the    epiphysis    in    these 
forms  is   comparable    only   to 
the   posterior   process   of   the 
"^—hf    Reptilia. 

In  addition  to  the  epiphy- 
sial evaginations,  another 
evagination  arises  from  the 
roof-plate  of  the  first  brain 
vesicle,  further  forward,  in 
the  region  which  becomes  the 
median  portion  of  the  telen- 
cephalon. This  paraiphysis,  as 
it  has  been  called,  has  been 
observed  in  the  lower  verte- 
brates and  in  the  Marsupials 
(Selenka),  but  up  to  the  pres- 
ent has  not  been  found  in 
other  groups  of  the  Mamma- 
lia. It  seems  to  be  compar- 
able to  a  chorioid  plexus 
which  is  evaginated  from  the 
brain  surface  instead  of 
being  invaginated  as  is  us- 
ually the  case.  There  is  no 
evidence  that  a  paraphysis  is 
developed  in  the  h  u  m  a  n 
brain. 

The  portion  of  the  roof- 
plate  which  lies  in  front  of 
the  epiphysis  represents  the 
velum  interpositnm  of  the 
adult  brain,  and  it  forms  at 
first  a  distinct  ridge  (Fig. 
232).  At  an  early  stage, 
however,  it  becomes  reduced 
to  a  thin  membrane  upon  the  sufrace  of  which  blood-vessels, 
developing  in  the  surrounding  mesenchyme,  arrange  them- 
selves at  about  the  third  month  in  two  longitudinal  plexuses. 


Fig.  232. — Dorsal  View  of  the 
Brain,  the  Roof  of  the  Lat- 
eral Ventricles  being  Re- 
moved, OF  an   Embryo  of   13.6 

MM. 

bj  Superior  brachiiim ;  eg,  lateral 
geniculate  body;  cfy,  chorioid 
plexus ;  cqa.  anterior  corpus 
quadrigeminum ;  h,  hippocam- 
pus;  hf,  hippocampal  fissure;  ot, 
thalamus;  p.  pineal  body;  7'p, 
roof-plate. —  (His.) 


THE    DIENCEPHALON.  423 

which,  with  the  subjacent  portions  of  the  vekim,  become 
invaginated  into  the  cavity  of  the  third  ventricle  to  form 
its  chorioid  plexus. 

The  dorsal  zones  thicken  in  their  more  dorsal  and  ante- 
rior portions  to  form  massive  structures,  the  thalami  (Figs. 
226,  V2,  and  232,  ot),  which,  encroaching  upon  the  cavity 
of  the  ventricle,  transform  it  into  a  narrow  slit-like  space, 
so  narrow,  indeed,  that  at  about  the  fifth  month  the  inner 
surfaces  of  the  two  thalami  come  in  contact  in  the  median 
line,  forming  what  is  known  as  the  intermediate  mass. 
More  ventrally  and  posteriorly  another  thickening  of  the 
dorsal  zone  occurs,  giving  rise  on  each  side  to  the  pidvinar 
of  the  thalamus  and  to  a  lateral  geniculate  body,  and  two 
ridges  extending  backward  and  dorsally  from  the  latter 
structures  to  the  thickenings  in  the  roof  of  the  mid-brain 
which  represent  the  anterior  corpora  quadrigemina,  give  a 
path  along  which  the  nerve-fibers  which  constitute  the  supe- 
rior quadrigeniinal  brachia  pass. 

From  the  ventral  zones  what  is  known  as  the  hypothala- 
mic region  develops,  a  mass  of  fibers  and  cells  whose  rela- 
tions and  development  are  not  yet  clearly  understood,  but 
which  may  be  regarded  as  the  forward  continuation  of  the 
tegmentum  and  reticular  formation.  In  the  median  line  of 
the  floor  of  the  ventricle  an  unpaired  thickening  appears, 
representing  the  corpora  mamillaria,  which  during  the  third 
month  becomes  divided  by  a  median  furrow  into  two  rounded 
eminences;  but  whether  these  structures  and  the  posterior 
portion  of  the  tuber  cinereum,  which  also  develops  from  this 
region  of  the  brain,  are  derivatives  of  the  ventral  zones  or 
of  the  floor-plate  is  as  yet  uncertain. 

Assuming  that  the  mamillaria  and  the  tuber  cinereum  are 
derived  from  the  ventral  zones,  the  origins  of  the  structures 
formed  from  the  walls  of  the  diencephalon  may  be  tabulated  as 
follows : 


424  THE    TELENCEPHALON. 

y^      r    ^  .  (  Velum  interpositum. 

Rooi-plate,  <  „  .  , 

I  Epiphysis. 

r  Thalami. 

Dorsal  zones,   <  Pulvinares. 

(^Lateral  geniculate  bodies. 

r  Hypothalamic  region. 

Ventral  zones, J  Corpora  mamillaria. 

1^  Tuber  cinereum  (in  part). 

Floor-plate,    Tissue  of  mid-ventral  line. 

The  Devclopnicnt  of  the  Telencephalon. — For  convenience 
of  description  the  telencephalon  may  be  regarded  as  con- 
sisting of  a  median  portion,  which  contains  the  anterior  part 
of  the  third  ventricle,  and  two  lateral  outgrowths  which  con- 
stitute the  cerebral  hemispheres.  The  roof  of  the  median 
portion  undergoes  the  same  transformation  as  does  the 
greater  portion  of  that  of  the  diencephalon  and  is  converted 
into  the  anterior  part  of  the  velum  interpositum  (Fig.  226, 
vi).  Anteriorly  this  passes  into  the  anterior  wall  of  the 
third  ventricle,  the  lamina  terminalis  (It),  a  structure  which 
is  to  be  regarded  as  formed  by  the  union  of  the  dorsal  zones 
of  opposite  sides,  since  it  lies  entirely  dorsal  to  the  anterior 
end  of  the  sulcus  Monroi.  From  the  ventral  part  of  the 
dorsal  zones  the  optic  evaginations  are  formed,  a  depression, 
the  optic  recess  (or),  marking  their  point  of  origin. 

The  ventral  zones  are  but  feebly  developed,  and  form  the 
anterior  part  of  the  hypothalamic  region,  while  at  the  ante- 
rior extremity  of  the  floor-plate  an  evagination  occurs,  the 
infundibular  recess  (ir),  which  elongates  to  form  a  funnel- 
shaped  structure  known  as  the  hypophysis.  At  its  extremity 
the  hypophysis  comes  in  contact  during  the  fifth  week  with 
the  enlarged  extremity  of  Rathke's  pouch  formed  by  an 
invagination  of  the  roof  of  the  oral  sinus  (see  p.  301),  and 
applies  itself  closely  to  the  posterior  surface  of  this  (Fig. 
219)  to  form  with  it  the  pituitary  body.  The  anterior  lobe 
at  an  early  stage  separates  from  the  mucous  membrane  of 


THE    TELENCEPHALON. 


425 


the  oral  sinus,  the  stalk  by  which  it  was  attached  completely 
disappearing,  and  toward  the  end  of  the  second  month  it  be- 
gins to  send  out  processes  from  its  walls  into  the  surrounding 
mesenchyme  and  so  becomes  converted  into  a  mass  of  solid 
epithelial  cords  embedded  in  a  mesenchyme  rich  in  blood  and 
lymphatic  vessels.  The  cords  later  on  divide  transversely 
to  a  greater  or  less  extent  to  form  alveoli,  the  entire  struc- 
ture coming  to  resemble  somewhat  the  parathyreoid  bodies 
(see  p.  315),  and,  like  these,  having  the  function  of  pro- 


FiG.  233. — Median  Longitudinal  Section  of  the  Brain  of  an  Em- 
bryo   OF    16.3    MM. 
br.    Anterior   brachium;    eg,    corpus    geniculatnm    laterale;    cs,    corpus 

striatum ;     h,    cerebral    hemisphere ;     ir,    infundibular     recess ;     It, 

lamina  terminalis ;  or,  optic  recess ;  ot,  thalamus ;  p,  pineal  process ; 

sm,  sulcus   Monroi ;   st,  hypothalamic  region ;   I'i,  velum   interposi- 

tum. —  (His.) 

ducing  an  internal  secretion.  The  posterior  lobe,  derived 
from  the  brain,  retains  its  connection  with  that  structure,  its 
stalk  being  the  mfimdibuhnn,  but  its  terminal  portion  does 
not  undergo  such  extensive  modifications  as  does  the  ante- 
rior lobe,  although  it  is  claimed  that  it  gives  rise  to  a  glandu- 
lar epithelium  which  may  become  arranged  so  as  to  form 
alveoli. 

The  cerebral  hemispheres  are   formed   from  the  lateral 
37 


426  THE    TELENCEPHALON. 

portions  of  the  dorsal  zones,  each  possessing  also  a  prolonga- 
tion of  the  roof -plate.  From  the  more  ventral  portion  of 
each  dorsal  zone  there  is  formed  a  thickening,  the  corpus 
striatum  (Figs.  233,  cs,  and  226,  VI  2),  a  structure  which 
is  for  the  telencephalon  what  the  optic  thalamus  is  for  the 
diencephalon,  while  from  the  more  dorsal  portion  there  is 
formed  the  remaining  or  mantle  (pallial)  portions  of  the 
hemispheres  (Figs.  233,  h,  and  226,  VI  4).  When  first 
formed,  the  hemispheres  are  slight  evaginations  from  the 
median  portion  of  the  telencephalon,  the  openings  by  which 
their  cavities  communicate  with  the  third  ventricle,  the  inter- 
vcutricular  foramina,  being  relatively  very  large  (Fig.  233), 
but,  in  later  stages  (Fig.  226),  the  hemispheres  increase 
more  markedly  and  eventually  surpass  all  the  other  portions 
of  the  brain  in  magnitude,  overlapping  and  completely  con- 
cealing the  roof  and  sides  of  the  diencephalon  and  mesen- 
cephalon and  also  the  anterior  surface  of  the  cerebellum. 
In  this  enlargement,  however,  the  interventricular  foramina 
share  only  to  a  slight  extent,  and  consequently  become  rela- 
tively smaller  (Fig.  226),  forming  in  the  adult  merely  slit- 
like openings  lying  between  the  lamina  terminalis  and  the 
thalami  and  having  for  their  roof  the  anterior  portion  of  the 
velum  interpositum. 

The  velum  interpositum, — that  is  to  say,  the  roof-plate, — 
where  it  forms  the  roof  of  the  interventricular  foramen,  is 
prolonged  out  upon  the  dorsal  surface  of  each  hemisphere, 
and,  becoming  invaginated,  forms  upon  it  a  groove.  As 
the  hemispheres,  increasing  in  height,  develop  a  mesial  wall, 
the  groove,  which  is  the  so-called  chorioidal  fissure,  comes 
to  lie  along  the  ventral  edge  of  this  wall,  and  as  the  growth 
of  the  hemispheres  continues  it  becomes  more  and  more  elon- 
gated, being  carried  at  first  backward  (Fig.  234),  then  ven- 
trally,  and  finally  forward  to  end  at  the  tip  of  the  temporal 
lobe.     After  the  establishment  of  the  e'rooves  the  mesen- 


THE    TELENCEPHALON. 


427 


chyme  in  their  vicinity  dips  into  them,  and,  developing  blood- 
vessels, becomes  the  chorioid  plexuses  of  the  lateral  ven- 
tricles, and  at  first  these  plexuses  grow  much  more  rapidly 
than  the  ventricles,  and  so  fill  them  almost  completely.  Later, 
however,  the  walls  of  the  hemispheres  gain  the  ascendancy 
in  rapidity  of  growth  and  the  plexuses  become  relatively 
much  smaller.  Since  the  portions  of  the  roof -plate  which 
form  the  chorioidal  fissures  are  continuous  with  the  velum 
interpositum  in  the  roofs  of  the  interventricular  foramina, 
the  chorioid  plexuses  of  the  lateral  and  third  ventricles  be- 
come continuous  also  at  that  point. 

The  mode  of  growth  of  the  chorioid  fissures  seems  to 
indicate  the  mode  of  growth  of 
the  hemispheres.  At  first  the 
growth  is  more  or  less  equal  in 
all  directions,  but  later  it  be- 
comes more  extensive  posteriorly, 
there  being  more  room  for  ex- 
pansion in  that  direction,  and 
when  further  extension  backward 
becomes  difficult  the  posterior  ex- 
tremities of  the  hemispheres  bend 
ventrally  toward  the  base  of  the 
cranium,  and  reaching  this,  turn 
forward  to  form  the  temporal 
lobes.  As  a  result  the  cavities  of 
the  hemispheres,  the  lateral  ven- 
tricles, in  addition  to  'being  carried  forward  to  form  an 
anterior  horn,  are  also  carried  backward  and  ventrally 
to  form  the  lateral  or  descending  horn,  and  the  corpus 
striatum  likewise  extends  backward  to  the  tip  of  each  tem- 
poral lobe  as  a  slender  process  known  as  the  tail  of  the 
caudate  nucleus.  In  addition  to  the  anterior  and  lateral 
horns,    the    ventricles    of    the    human    brain    also    possess 


Fig.  234. — Median  Longi- 
tudinal Section  of  the 
Brain  of  an  Embryo 
Calf  of  5  cm. 

ch,  Cerebellum ;  cp,  chorioid 
plexus ;  cs,  corpus  stria- 
tum; fM,  interventricular 
foramen  ;  in,  hypophysis  ; 
m,  xnid-brain ;  oc,  optic 
commissure ;  t,  posterior 
part  of  the  diencephalon. 
—  {Mihalkovics.) 


428  THE    TELENCEPHALON. 

posterior  horns  extending  backward  into  the  occipital  por- 
tions of  the  hemispheres,  these  portions,  on  account  of  the 
greater  persistence  of  the  mid-brain  flexure  (see  p.  412), 
being  enabled  to  develop  to  a  greater  extent  than  in  the  lower 
mammals. 

The  scheme  of  the  origin  of  parts  in  the  telencephalon 
may  be  stated  as  follows  : 

Median  Part.  Hemispheres. 

_      ^    ,  (  Anterior  part  of  Velum  r  Floor   of   chorioidal   fis- 

Roof-plate,    \      .  ^  .^  \ 

{     mterpositum.  I      sure. 

r  Pallium. 
f  Lamina  terminalis.  I  Corpus  striatum. 

'   "'  I  Optic  evaginations.  1    Olfactory    lobes    (see   p. 

[      433)- 

f  Anterior   part    of   hypo- 

..^         ,  I      thalamic  region. 

Ventral  zones,  ..<    ,    ^    ■  .       r    ,   , 

Anterior    part    of    tuber 

[^     cinereum. 

The  Convolutions  of  the  Hemispheres. — The  growth  of 
the  hemispheres  to  form  the  voluminous  structures  found 
in  the  adult  depends  mainly  upon  an  increase  of  size  of  the 
pallium.  The  corpus  striatum,  although  it  takes  part  in 
the  elongation  of  each  hemisphere,  nevertheless  does  not 
increase  in  other  directions  as  rapidly  and  extensively  as 
the  pallium,  and  hence,  even  in  very  early  stages,  a  depres- 
sion appears  upon  the  surface  of  the  hemispheres  where  the 
corpus  is  situated  (Fig.  275).  This  depression  is  the  lateral 
cerebral  fossa,  and  for  a  considerable  period  it  is  the  only 
sign  of  inequality  of  growth  on  the  outer  surfaces  of  the 
hemispheres.  Upon  the  mesial  surfaces,  however,  at  about 
the  time  that  the  chorioid  fissure  appears,  another  linear 
depression  is  formed  dorsal  to  the  chorioid,  and  when  fully 
formed  extends  from  in  front  of  the  interventricular  fora- 
men to  the  tip  of  the  temporal  lobe  (Fig.  237,  h).     It  affects 


THE    CEREBRAL    CONVOLUTIONS. 


429 


from  the 
where  it 
continued 


the  entire  thickness  of  the  palHal  wall  and  consequently  pro- 
duces an  elevation  upon  the  inner  surface,  a  projection  into 
the  cavity  of  the  ventricle  which  is  known  as  the  hippocam- 
pus, whence  the  fissure  may  be  termed  the  hippocampal  fis- 
sure. The  portion  of  the  pallium  which  intervenes  between 
this  fissure  and  the  chorioidal  forms  what  is  known  as  the 
dentate  gyrus. 

Toward  the  end  of  the  third  or  the  beginning  of  the  fourth 
month  two  prolonga 
tions  arise 
fissure  just 
turns  to  be 
into  the  temporal  lobe, 
and  these,  extending 
posteriorly,  give  rise 
to  the  parieto-occipital 
and  calcarine  fissures. 
Like  the  hippocampal, 
these  fissures  produce 
elevations  upon  the  in- 
ner surface  of  the  pal- 
lium, that  formed  by 
the  parieto -occipital 
early  disappearing,  while  that  produced  by  the  calcarine 
persists  to  form  the  calcar  (^hippocampus  minor)  of  adult 
anatomy. 

The  three  fissures  just  described,  together  with  the 
chorioidal  and  the  lateral  cerebral  fossa,  are  all  formed  by 
the  beginning  of  the  fourth  month  and  all  affect  the  entire 
thickness  of  the  wall  of  the  hemisphere,  and  hence  have 
been  termed  the  primary  or  total  fissures.  Until  the  begin- 
ning of  the  fifth  month  they  are  the  only  fissures  present, 
but  at  that  time  secondary  fissures,  which,  with  one  excep- 
tion,  are  merely   furrows   of  the   surface  of  the  pallium, 


Fig.  235. — Bbain  of  an  Embryo  of  the 

Fourth    Month. 
c,   Cerebellum ;  p,  pons,  s,  lateral  cere- 
bra  fossa. 


430 


The  cerebral  convolutions. 


make  their  appearance  and  continue  to  form  until  birth 
and  possibly  later.  Before  considering"  these,  however, 
certain  changes  which  occur  in  the  neig'hborhood  of  the 
lateral  cerebral  fossa  may  be  described. 

The  fossa  is  at  first  a  triangular  depression  situated 
above  the  temporal  lobe  on  the  surface  of  the  hemisphere. 
During  the  fourth  month  it  deepens  considerably,  so  that 
its  upper  and  lower  margins  become  more  pronounced  and 


pic 


Fig.  236. — Cerebral  Hemisphere  of  an  Embryo  of  about  the  Seventh 

Month. 

s,  Superior  frontal  sulcus ;  ip,  interparietal ;  IR,  insula ;  pel,  inferior 
pre-central ;  pes,  superior  pre-central ;  ptc,  post-central;  R,  central; 
.S",  lateral;  t,  first  temporal. —  (Cunningham.) 


form  projecting  folds,  and,  during  the  fifth  month,  these 
two  folds  approach  one  another  and  eventually  cover  in  the 
floor  of  the  fossa  completely,  the  groove  which  marks  the 
line  of  their  contact  forming  the  lateral  cerebral  fissure, 
while  the  floor  of  the  fossa  becomes  known  as  the  insula. 

The  first  of  the  secondary  fissures  to  appear  is  the  sulcus 
ciiii^iili,  which  is  formed  about  the  middle  of  the  fifth  month 


THE    CORPUS    CALLOSUM.  43  I 

on  the  mesial  surface  of  the  hemispheres,  lying  parallel  to 
the  anterior  portion  of  the  hippocampus  fissure  and  divid- 
ing the  mesial  surface  into  the  gyri  marginalis  and  forni- 
cotus.  A  little  later,  at  the  beginning  of  the  sixth  month, 
several  other  fissures  make  their  appearance  upon  the  outer 
surface  of  the  pallium,  the  chief  of  these  being  the  central 
sulcus,  the  inter-parietal,  the  pre-  and  post-central,  and  the 
temporal  sulci,  the  most  ventral  of  these  last  running  paral- 
lel with  the  lower  portion  of  the  hippocampal  fissure  and 
differing  from  the  others  in  forming  a  ridge  on  the  wall 
of  the  ventricle  termed  the  collateral  eminence,  whence  the 
fissure  is  known  as  the  collateral.  The  position  of  most  of 
these  fissures  may  be  seen  from  Fig.  236,  and  for  a  more 
complete  description  of  them  reference  may  be  had  to  text- 
books of  descriptive  anatomy. 

In  later  stages  numerous"  tertiary  fissures  make  their 
appearance  and  mask  more  or  less  extensively  the  seconda- 
ries, than  which  they  are,  as  a  rule,  much  more  inconstant 
in  position  and  shallower. 

The  Corpus  Callosmn  and  Fornix. — While  these  fissures 
have  been  forming,  important  structures  have  developed 
in  connection  with  the  lamina  terminalis.  Up  to  about  the 
fourth  month  the  lamina  is  thin  and  of  nearly  uniform 
thickness  throughout,  but  at  this  time  it  begins  to  thicken 
near  its  dorsal  edge  and  fibers  appear  in  the  thickening, 
converting  it  into  the  anterior  commissure.  Immediately 
above  and  in  front  of  the  upper  edge  of  the  lamina  ter- 
minalis the  medial  walls  of  the  two  cerebral  hemispheres 
come  into  contact  and  fuse,  and  the  area  of  fusion  soon 
becomes  continuous  with  the  thickened  upper  edge  of  the 
lamina  (Fig.  237).  In  later  stages  the  area  of  concres- 
cence of  the  hemispheres  extends  both  anteriorly  and  pos- 
teriorly and  assumes  the  form  of  a  triangle  with  its  apex 
directed  backwards.     In  the  dorsal  portion  of  the  triangle 


432 


THE    CORPUS    CALLOSUM. 


fibers  extend  across  from  the  pallium  of  one  hemisphere  to 
that  of  the  other  and  form  the  corpus  callosum  (Fig.  238), 
while  in  its  ventral  edge  other  fibers  extend  from  the  hippo- 
campus to  the  lamina  terminalis,  and,  descending  in  that 
structure,  pass  posteriorly  in  the  floor  of  the  third  ventricle 
toward  the  corpora  mamillaria.     These  fibers  constitute  the 

fornix,  whose  peculiar 
course  in  the  adult 
brain  may  be  under- 
stood by  a  consideration 
of  the  rotation  of  the 
hemispheres  during 
growth  which  results 
in  the  formation  of  the 
temporal  lobe  (see  p. 
427). 

The   portion   of  the 
triangle    included    be- 
tween the  callosum  and 
the  fornix  remains  thin 
and  forms  the  septum 
pelluciduni,  and  a  split 
occurring  in  the  center 
of  this  gives  rise  to  the 
so-called  fifth  ventricle, 
which,   from  its  mode 
of  formation,  is  a  com- 
pletely closed  cavity  and  is  not  lined  with  ependymal  tissue 
of    the    same    nature    as    that    found    in    the    other    ven- 
tricles. 

Owing  to  the  very  considerable  size  reached  by  the  area 
of  concrescence  of  the  hemispheres,  whose  history  has  just 
been  described,  important  changes  are  wrought  in  the  ad- 
joining portions  of  the  mesial  surface  of  the  hemispheres. 
Before  the  development  of  the  area  the  gyrus  dentatus  and 


Fig.  237. — Median  Longitudinal  Sec- 
tion OF  THE  Brain  of  an  Embryo  of 
Four   Months. 

c,  Calcarine  fissure;  ca,  anterior  commis- 
sure ;  cc,  corpus  callosum ;  cf,  chori- 
oidal  fissure;  dg,  dentate  gyrus;  fm, 
interventricular  foramen ;  h,  hippo- 
canipal  fissure;  po,  parieto-occipital 
fissure. —  {Mihalkovicz.) 


THE    CORPUS    CALLOSUM. 


433 


the  hippocampus  extend  forward  into  the  anterior  portion 
of  the  hemispheres  (Fig.  237),  but  on  account  of  their  posi- 
tion they  become  encroached  upon  by  the  enlargement  of 
the     corpus     callosum, 

with    the    resuh    that  vi 

the  hippocampus  be- 
comes practically  ob- 
literated in  that  por- 
tion of  its  course 
which  lies  in  the  re- 
gion occupied  by  the 
corpus  callosum,  its 
fissure  in  this  region 
becoming  known  as 
the  callosal  fissure, 
while  the  correspond- 
ing portions  of  the 
dentate  gyrus  become 
reduced  to  narrow 
and  insignificant  bands 
of  nerve-tissue  which  rest  upon  the  upper  surface  of  the 
corpus  callosum  and  are  known  as  the  lateral  longitudinal 
strice. 

Some  doubt  still  exists  as  to  the  exact  mode  of  formation 
of  the  fifth  ventricle.  Some  authors  maintain  that  it  is  a  por- 
tion of  the  longitudinal  fissure  of  the  cerebrum,  separated  from 
the  rest  by  the  forward  growth  of  the  corpus  callosum  and 
finally  closed  by  the  secondary  union  of  the  rostrum  with  the 
lamina  terminalis. 

The  Olfactory  Lobes. — At  the  time  when  the  cerebral 
hemispheres  begin  to  enlarge — that  is  to  say,  at  about  the 
fourth  week — a  slight  furrow,  which  appears  on  the  ven- 
tral surface  of  each  anteriorly,  marks  off  an  area  which, 
continuing  to  enlarge  with  the  hemispheres,  gradually  be- 
comes constricted  ofif  from  them  to  form  a  distinct  lobe-like 

38 


Fig.  238. — Median  Longitudinal  -Sec- 
tion OF  THE  Brain  of  an  Embryo  of 
THE  Fifth  Month. 

ac.  Anterior  commissure ;  cc,  corpus  cal- 
losum ;  dg,  dentate  gyrus ;  f,  fornix ; 
i,  infundibulum ;  mc,  intermediate 
mass ;  si,  septum  pellucidum ;  vi,  velum 
interpositum. —  (Mihalkovics.) 


434  THE    OLFACTORY    LOBES. 

structure,  the  olfactory  lobe  (Fig.  219,  VI  3).  In  most 
of  the  lower  mammaHa  these  lobes  reach  a  very  consider- 
able size,  and  consequently  have  been  regarded  as  consti- 
tuting an  additional  division  of  the  brain,  known  as  the 
rhinencephalon,  but  in  man  they  remain  smaller,  and  al- 
though they  are  at  first  hollow,  containing  prolongations 
from  the  lateral  ventricles,  the  cavities  later  on  disappear 
and  the  lobes  become  solid.  Each  lobe  becomes  differen- 
tiated into  two  portions,  its  terminal  portion  becoming  con- 
verted into  the  club-shaped  structure,  the  olfactory  bulb  and 
stalk,  while  its  proximal  portion  gives  rise  to  the  olfactory 
tracts,  the  trigone,  and  the  anterior  perforated  substance. 

Histogenesis  of  the  Cerebral  Cortex. — A  satisfactory 
study  of  the  histogenesis  of  the  cortex  has  not  yet  been 
made.  In  embryos  of  three  months  a  marginal  velum  is 
present  and  probably  gives  rise  to  the  stratum  zonale  of 
the  adult  brain;  beneath  this  is  a  cellular  layer,  perhaps 
representing  the  mantle  layer;  beneath  this,  again,  a  layer 
of  nerve-fibers  is  beginning  to  appear,  representing-  the 
white  substance  of  the  pallium;  and,  finally,  lining  the 
ventricle  is  an  ependymal  layer.  In  embryos  of  the  fifth 
month,  toward  the  innermost  part  of  the  second  layer,  cells 
are  beginning  to  differentiate  into  the  large  pyramidal  cells, 
but  almost  nothing  is  known  as  to  the  orig^in  of  the  other 
layers  recognizable  in  the  adult  cortex,  nor  is  it  known 
whether  any  migration,  similar  to  what  occurs  in  the  cere- 
bellar cortex,  takes  place.  The  fibers  of  the  white  sub- 
stance do  not  begin  to  acquire  their  myelin  sheaths  until 
toward  the  end  of  the  ninth  month,  and  the  process  is  not 
completed  until  some  time  after  birth  (Flechsig),  while 
the  fibers  of  the  cortex  continue  to  undergo  myelination 
until  comparatively  late  in  life  (Kaes). 

The  Development  of  the  Spinal  Nerves. — It  has  already 
been  seen  that  there  is  a  fundamental  difference  in  the  mode 


THE    SPINAL    NERVES.  435 

of  development  of  the  two  roots  of  which  the  typical  spinal 
nerves  are  composed,  the  ventral  root  being  formed  by  axis- 
cylinders  which  arise  from  neuroblasts  situated  within  the 
substance  of  the  spinal  cord,  while  the  dorsal  roots  arise 
from  the  cells  of  the  neural  crests,  their  axis-cylinders  grow- 
ing into  the  substance  of  the  cord  while  their  dendrites 
become  prolonged  peripherally  to  form  the  sensory  fibers 
of  the  nerves.  Throughout  the  thoracic,  lumbar  and  sacral 
regions  of  the  cord  the  fibers  which  grow  out  from  the 
anterior  horn  cells  converge  to  form  a  single  nerve-root  in 
each  segment,  but  in  the  cervical  region  fibers  which  arise 
from  the  more  laterally  situated  neuroblasts  make  their  exit 
from  the  cord  independently  of  the  more  ventral  neuroblasts 
and  form  the  roots  of  the  spinal  accessory  nerve  (see  p 
444).  In  the  cervical  region  there  are  accordingly  three 
sets  of  nerve-roots,  the  dorsal,  lateral,  and  ventral  sets. 

In  a  typical  spinal  nerve,  ■  such  as  one  of  the  thoracic 
series,  the  dorsal  roots  as  they  grow  peripherally  pass  down- 
ward as  well  as  outward,  so  that  they  c|uickly  come  into  con- 
tact with  the  ventral  roots  with  whose  fibers  they  mingle, 
and  the  mixed  nerve  so  formed  soon  after  divides  into  two 
trunks,  a  dorsal  one,  which  is  distributed  to  the  dorsal  mus- 
culature and  integument,  and  a  larger  ventral  one.  The 
ventral  division  as  it  continues  its  outward  growth  soon 
reaches  the  dorsal  angle  of  the  pleuro-peritoneal  cavity, 
where  it  divides,  one  branch  passing  into  the  tissue  of  the 
body-wall  while  the  other  passes  into  the  splanchnic  meso- 
derm. The  former  branch,  continuing  its  onward  course 
in  the  body-wall,  again  divides,  one  branch  becoming  the 
lateral  cutaneous  nerve,  while  the  other  continues  inward 
to  terminate  in  the  median  ventral  portion  of  the  body  as 
the  anterior  cutaneous  nerve.  The  splanchnic  branch  forms 
a  ramus  communicans  to  the  sympathetic  system  and  will  be 
considered  more  fully  later  on. 


43^  THE    CRANIAL    NERVES. 

The  conditions  just  described  are  those  which  obtain 
throughout  the  greater  part  of  the  thoracic  region.  Else- 
where the  fibers  of  the  ventral  divisions  of  the  nerves  as 
they  grow  outward  tend  to  separate  from  one  another  and 
to  become  associated  with  the  fibers  of  adjacent  nerves, 
giving  rise  to  plexuses.  In  the  regions  where  the  limbs 
occur  the  formation  of  the  plexuses  is  also  associated  with 
a  shifting  of  the  parts  to  which  the  nerves  are  supplied,  a 
factor  in  plexus  formation  which  is,  however,  much  more 
evident  from  comparative  anatomical  than  from  embryo- 
logical  studies. 

The  Development  of  the  Cranial  Nerves. — During  the 
last  thirty  years  the  cranial  nerves  have  received  a  great 
deal  of  attention  in  connection  with  the  idea  that  an  accu- 
rate knowledge  of  their  development  would  afford  a  clue 
to  a  most  vexed  problem  of  vertebrate  morphology,  the 
metamerism  of  the  head.  That  the  metamerism  which  was 
so  pronounced  in  the  trunk  should  extend  into  the  head  was 
a  natural  supposition,  strengthened  by  the  discovery  of 
head-cavities  in  the  lower  vertebrates  and  by  the  indica- 
tions of  metamerism  seen  in  the  branchial  arches,  and  the 
problem  which  presented  itself  was  the  correlation  of  the 
various  structures  belonging  to  each  metamere  and  the  de- 
termination of  the  modifications  which  they  had  undergone 
during  the  evolution  of  the  head. 

In  the  trunk  region  a  nerve  forms  a  conspicuous  element 
of  each  metamere  and  is  composed,  according  to  what  is 
known  as  Bell's  law,  of  a  ventral  or  efferent  and  a  dorsal 
or  afferent  root.  Until  comparatively  recently  the  study  of 
the  cranial  nerves  has  been  dominated  by  the  idea  that 'it 
was  possible  to  extend  the  application  of  Bell's  law  to  them 
and  to  recognize  in  the  cranial  region  a  number  of  nerve 
pairs  serially  homologous  with  the  spinal  nerves,  some  of 
them,  however,  having  lost  their  afferent  roots,  while  in 


THE    CRANIAL    NERVES.  437 

Others  a  dislocation,  as  it  were,  of  the  two  roots  had  oc- 
curred. 

The  results  obtained  from  investigation  along  this  line 
have  not,  however,  proved  entirely  satisfactory,  and  facts 
have  been  elucidated  which  seem  to  show  that  it  is  not 
possible  to  extend  Bell's  law,  in  its  original  form  at  least, 
to  the  cranial  nerves.  It  has  been  found  that  it  is  not 
sufficient  to  recognize  simply  afferent  and  efferent  roots, 
but  these  must  be  analyzed  into  further  components,  and 
when  this  is  done  it  is  found  that  in  the  series  of  cranial 
nerves  certain  components  occur  which  are  not  represented 
in  the  nerves  of  the  spinal  series. 

Before  proceeding  to  a  description  of  these  components 
it  will  be  well  to  call  attention  to  a  matter  already  alluded 
to  in  a  previous  chapter  (p.  no)  in  connection  with  the 
segmentation  of  the  mesoderm  of  the  head.  It  has  been 
pointed  out  that  while  there  exist  "  head-cavities  "  which 
are  serially  homologous  with  the  mesodermal  somites  of 
the  trunk,  there  has  been  imposed  upon  this  primary  cranial 
metamerism  a  secondary  metamerism  represented  by  the 
branchiomeres  associated  with  the  branchial  arches,  and, 
it  may  be  added,  this  secondary  metamerism  has  become 
the  more  prominent  of  the  two,  the  primary  one,  as  it  devel- 
oped, gradually  slipping  into  the  background  until,  in  the 
higher  vertebrates,  it  has  become  to  a  very  considerable 
extent  rudimentary.  In  accordance  with  this  double  me- 
tamerism it  is  necessary  to  recognize  two  sets  of  cranial 
muscles,  one  derived  from  the  cranial  myotomes  and  rep- 
resented by  the  muscles  of  the  eyeball,  and  one  derived  from 
the  branchiomeric  mesoderm,  and  it  is  necessary  also  to 
recognize  for  these  two  sets  of  muscles  two  sets  of  motor 
nerves,  so  that,  with  the  dorsal  or  sensory  nerve-roots,  there 
are  altogether  three  sets  of  nerve-roots  in  the  cranial  region 
instead  of  only  two,  as  in  the  spinal  region. 


438 


THE    CRANIAL    NERVES. 


These  three  sets  of  roots  are  readily  recognizable  both 
in  the  embryonic  and  in  the  adult  brain,  especially  if  atten- 
tion be  directed  to  the  cell  groups  or  nuclei  with  which  they 
are  associated  (Fig.  239).  Thus  there  can  be  recognized: 
( I )  a  series  of  nuclei  from  which  nerve-fibers  arise,  situ- 
ated in  the  floor  of  the  fourth  ventricle  and  iter  close  to  the 
median  line  and  termed  the  ventral  motor  nuclei;  (2)  a 
second  series  of  nuclei  of  origin,  situated  more  laterally  and 


Fig.  239. — Transverse  Section  through  the  Medulla  Oblongata 
OF  an  Embryo  of  10  mm.,  showing  the  Nuclei  of  Origin  of  the 
Vagus   {X)  and  Hypoglossal  (A7/)   Nerves. —  {His.) 

in  the  substance  of  the  formatio  reticularis,  and  known  as 
the  lateral  motor  nuclei;  and  (3)  a  series  of  nuclei  in  which 
afferent  nerve-fibers  terminate,  situated  still  more  laterally 
in  the  floor  of  the  ventricle  and  forming  the  dorsal  or  sen- 
sory nuclei.  None  of  the  twelve  cranial  nerves  usually 
recognized  in  the  text-books  contains  fibers  associated  with 
all  three  of  these  nuclei ;  the  fibers  from  the  lateral  motor 


THE    CRANIAL    NERVES. 


439 


nuclei  almost  invariably  unite  with  sensory  fibers  to  form 
a  niixed  nerve,  but  those  from  all  the  ventral  motor  nuclei 
form  independent  roots,  while  the  olfactory  and  auditory 
nerves  alone,  of  all  the  sensory  roots  (omitting  for  the  pres- 
ent the  optic  nerve),  do  not  contain  fibers  from  either  of 
the  series  of  motor  nuclei.  The  relations  of  the  various 
cranial  nerves  to  the  nuclei  may  be  seen  from  the  following 
table,  in  which  the  -j-  sign  indicates  the  presence  and  the  — 
sign  the  absence  of  fibers  from  the  nuclear  series  under 
which  it  stands : 


Number. 

Name. 

Ventral 
Motor. 

Lateral 
Motor. 

Sensory. 

T. 
III. 

Olfactory. 
Oculomotor. 

+ 

— 

+ 

IV. 

Trochlear. 

+ 

— 

— 

V. 

VI. 

Trigeminus. 
Abducens. 

+ 

+ 

+ 

VII. 

VIII. 

IX. 

X. 

XI. 

Facial. 
Auditory. 
Glossopharyngeal. 
Vagus.                      "I 
Spinal  Accessory.  ( 

+ 
+ 

+ 
+ 

+ 

+ 

Two  nerves — namely,  the  second  and  twelfth — have  been 
omitted  from  the  above  table.  Of  these,  the  second  or  optic 
nerve  undoubtedly  belongs  to  an  entirely  different  category 
from  the  other  peripheral  nerves,  and  will  be  considered  in 
the  following  chapter  in  connection  with  the  sense-organ 
with  which  it  is  associated  (see  especially  p.  491).  The 
twelfth  or  hypoglossal  nerve,  on  the  other  hand,  really  be- 
longs to  the  spinal  series  and  has  only  secondarily  been  taken 
up  into  the  cranial  region  in  the  higher  vertebrates.  It  has 
already  been  seen  (p.  179)  that  the  bodies  of  four  vertebrae 
are  included  in  the  basioccipital  bone,  and  that  three  of  the 
nerves  corresponding  to  these  vertebrae  are  represented  in 
the  adult  by  the  hypoglossal  and  the  fourth  by  the  first  cer- 
vical or  suboccipital  nerve.     The  dorsal  roots  of  the  hypo- 


440  THE    CRANIAL    NERVES. 

glossal  nerves  seem  to  have  almost  disappeared,  althong-h  a 
ganglion  has  been  observed  in  embryos  of  7  and  10  mm.  in 
the  posterior  part  of  the  hypoglossal  region  (His),  and  prob- 
ably represents  the  dorsal  root  of  the  most  posterior  portion 
of  the  hypoglossal  nerve.  This  ganglion  disappears,  as  a 
rule,  in  later  stages,  and  it  is  interesting  to  note  that  the 
ganglion  of  the  suboccipital  nerve  is  also  occasionally  want- 
ing in  the  adult  condition.  The  hypoglossal  roots  are  to 
be  regarded,  then,  as  equivalent  to  the  ventral  roots  of  the 
cervical  spinal  nerves,  and  the  nuclei  from  which  they  arise 
lie  in  series  with  the  cranial  ventral  motor  roots,  a  fact 
which  indicates  the  equivalency  of  these  latter  with  the  fibers 
which  arise  from  the  neuroblasts  of  the  anterior  horns  of 
the  spinal  cord. 

The  equivalents  of  the  lateral  motor  roots  may  more  con- 
veniently be  considered  later  on,  but  it  may  be  pointed  out 
here  that  these  are  the  fibers  which  are  distributed  to  the 
muscles  of  the  branchiomeres.  In  the  case  of  the  sensory 
nerves  a  further  analysis  is  necessary  before  their  equiva- 
lents in  the  spinal  series  can  be  determined.  For  this  the 
studies  which  have  been  made  in  recent  years  of  the  com- 
ponents entering  into  the  cranial  nerves  of  the  amphibia 
(Strong)  and  fishes  (Herrick)  must  supply  a  basis,  since 
as  yet  a  direct  analysis  of  the  mammalian  nerves  has  not 
been  made.  In  the  forms  named  it  has  been  found  that 
three  diiTerent  components  enter  into  the  formation  of  the 
dorsal  roots  of  the  cranial  nerves :  ( i )  fibers  belonging  to 
a  general  cutaneous  or  somatic  sensory  system^  distributed 
to  the  skin  without  being  connected  with  any  special  sense- 
organs;  (2)  fibers  belonging  to  what  is  termed  the  com- 
munis or  viscera-sensory  system^  distributed  to  the  walls  of 
the  mouth  and  pharyngeal  region  and  to  special  organs 
found  in  the  skin  of  the  same  character  as  those  occurring 
in  the  mouth;  and  (3)  fibers  belonging  to  a  special  set  of 


THE    CRANIAL    NERVES.  44 1 

cutaneous  sense-organs  largely  developed  in  the  fishes  and 
known  as  the  organs  of  the  lateral  line. 

The  fibers  of  the  somatic  sensory  system  converge  to  a 
group  of  cells,  situated  in  the  lateral  part  of  the  floor  of 
the  fourth  ventricle  and  forming  what  is  termed  the  tri- 
geminal lobe,  and  also  extend  posteriorly  in  the  substance 
of  the  medulla  (Fig.  240),  forming  what  has  been  termed 
the  spinal  root  of  the  trigeminus  and  terminating  in  a 
column  of  cells  which  represents  the  forward  continuation 
of  the  posterior  horn  of  the  cord.  In  the  fishes  and  am- 
phibia fibers  belonging  to  this  system  are  to  be  found  in  the 
fifth,  seventh,  and  tenth  nerves,  but  in  the  mammalia  their 
distribution  has  apparently  become  more  limited,  being  con- 
fined almost  exclusively  to  the  trigeminus,  of  whose  sensory 
divisions  they  form  a  very  considerable  part.  Since  the 
cells  around  which  the  fibers  of  the  spinal  root  of  the  tri- 
geminus terminate  are  the  forward  continuations  of  the  pos- 
terior horns  of  the  cord,  it  seems  probable  that  the  fibers  of 
this  system  are  the  cranial  representatives  of  the  posterior 
roots  of  the  spinal  nerves,  which,  it  may  be  noted,  are  also 
somatic  in  their  distribution. 

The  fibers  of  the  viscero-sensory  system  are  found  in  the 
lower  forms  principally  in  the  ninth  and  tenth  nerves  (see 
Fig.  240),  although  groups  of  them  are  also  incorporated 
in  the  seventh  and  fifth.  They  converge  to  a  mass  of  cells, 
known  as  the  lobus  vagi,  and  like  the  first  set  are  also  con- 
tinued down  the  medulla  to  form  a  tract  known  as  the  fas- 
ciculus  solitarius  or  fasciculus  communis.  In  the  mammalia 
the  system  is  represented  by  the  sensory  fibers  of  the  glosso- 
pharyngeo-vagus  set  of  nerves,  of  which  it  represents  prac- 
tically the  entire  mass;  by  the  sensory  fibers  of  the  facial 
arising  from  the  geniculate  ganglion  and  included  in  the 
chorda  tympani  and  probably  also  the  great  superficial 
petrosal;  and  also,  probably,  by  the  lingual  branch  of  the 


442 


THE    CRANIAL    NERVES. 


trigeminus.  Furthermore,  since  the  mucous  membrane  of 
the  palate  is  supphed  by  branches  from  the  trigeminus  which 
pass  by  way  of  the  spheno-palatine  (Aleckel's)  ganghon, 
and  the  same  region  is  supphed  in  lower  forms  by  a  palatine 
branch  from  the  facial,  it  seems  probable  that  the  palatine 


rl^ 


cc 


rix 


Fig.  240. — Diagrams  showing  the  Sensory  Components  of  the 
Cranial  Nerves  of  a  Fish    (Menidia). 

The  somatic  sensory  system  is  unshaded,  the  viscero-sensory  is  cross- 
hatched,  and  the  lateral  line  system  is  black.  asc.v.  Spinal 
root  of  trigeminus ;  brx,  branchial  branches  of  vagus ;  ol,  olfactory 
bulb;  op,  optic  nerve;  rc.x,  cutaneous  branch  of  the  vagus;  rix, 
intestinal  branch  of  vagus ;  rl.  lateral  line  nerve ;  rl.acc,  accessory 
lateral  line  nerve;  ros,  superficial  ophthalmic;  rp,  ramus  palatinus 
of  the  facial;  thy,  hyomandibular  branch  of  the  facial;  t.inf,  infra- 
orbital nerve. —  {Her  rick.) 


nerves  of  the  mammalia  are  also  to  be  assigned  to  this  sys- 
tem.*    If  this  be  the  case,  a  very  evident  clue  is  afforded  to 

*  The  fact  that  the  palatine  branches  are  associated  with  the  tri- 
geminus in  the  Mammalia  and  with  the  facial  in  the  Amphibia  is  readily 
explained  by  the  fact  that  in  the  latter  the  Gasserian  and  geniculate 
ganglia  are  not  always  separated,  so  that  it  is  possible  for  fibers  origi- 
nating from  the  compound  ganglion  to  pass  into  either  nerve. 


THE  CRANIAL  XERVES.  443 

the  hijmologies  of  the  system  in  the  spinal  nerves,  for  since 
the  spheno-palatine  ganghon  is  to  be  regarded  as  part  of 
the  sympathetic  system,  the  sensory  fibers  which  pass  from 
the  viscera  to  the  spinal  cord  by  way  of  the  sympathetic 
system  (p.  448)  present  relations  practically  identical  with 
those  of  the  palatine  nerves. 

Finally,  with  regard  to  the  system  of  the  lateral  line,  there 
seems  but  little  doubt  that  it  has  no  representation  whatso- 
ever in  the  spinal  nerves.  It  is  associated  with  a  peculiar 
system  of  cutaneous  sense-organs  found  only  in  aquatic  or 
marine  animals,  and  also  with  the  auditory  and  possibly  the 
olfactory  organs,  the  former  of  which  are  certainl}^  and  the 
latter  possibly  primarily  parts  of  the  lateral  line  system  of 
organs.  The  organs  are  principalh'  confined  to  the  head, 
although  they  also  extend  upon  the  trunk,  where  they  are 
followed  by  a  branch  from  the  vagus  nerve,  the  entire  system 
being  accordingly  supplied  by  cranial  nerves.  In  the  fishes, 
in  which  the  development  of  the  organs  is  at  a  maximum, 
fibers  belonging  to  the  system  are  found  in  all  the  branchio- 
meric  ners'es  and  all  converge  to  a  portion  of  the  medulla 
known  as  the  tuhcrculnm  aaisficum.  In  the  ^Mammalia, 
with  the  disappearance  of  the  lateral  line  organs  there  has 
been  a  disappearance  of  the  associated  nerves,  and  the  only 
certain  representative  of  the  system  which  persists  is  the 
auditory  nerve. 

The  table  given  on  page  439  may  now  be  expanded  as 
follows,  though  it  must  be  recognized  that  such  an  analysis 
of  the  mammalian  nerves  is  merely  a  deduction  from  what 
has  been  observed  in  lower  forms,  and  may  require  some 
modifications  when  the  components  have  been  sul^jected  to 
actual  observation : 


444 


THE    CRANIAL    NERVES. 


Nerve. 

Ventral 
M  otor. 

Lateral 
Motor. 

Somatic 
Sensory. 

Visceral 
Sensory. 

Lateral 
Line. 

I. 

_ 



_ 

_ 

+ 

III. 

+ 

— 

— 

— 

IV. 

+ 

— 

— 

— 

— 

V. 
VI. 

+ 

+ 

+ 

+ 

— 

VII. 
VIII 

+ 

— 

+ 

+■ 

IX.) 

X. 
XI.  i 

— 

+ 

+ 

+ 

— 

XII. 

+ 

— 

— 

— 

— 

Spinal. 

+ 

(?) 

+ 

H- 

— 

An  additional  word  is  necessary  concerning  the  spinal 
accessory  nerve,  for  it  presents  certain  interesting  relations 
which  possibly  furnish  a  clue  to  the  spinal  equivalents  of  the 
lateral  motor  roots.  In  the  first  place,  the  neuroblasts  which 
give  rise  to  those  fibers  of  the  nerve  which  come  from  the 
spinal  cord  are  situated  in  the  dorsal  part  of  the  ventral 
zones  and  in  the  adult  in  the  lateral  horn  of  the  cord.  As 
the  nuclei  of  origin  are  traced  anteriorly  they  will  be  found 
to  change  their  position  somewhat  as  the  medulla  is  reached 
and  eventually  come  to  lie  in  the  reticular  formation,  the 
most  anterior  of  them  being  practically  continuous  with  the 
motor  nucleus  of  the  vagus.  Indeed,  it  seems  that  the 
spinal  accessory  nerve  is  properly  to  be  regarded  as  an  ex- 
tension of  the  vagus  downward  into  the  cervical  region 
(Fiirbringer,  Streeter),  a  process  which  reaches  its  greatest 
development  in  the  mammalia  and  seems  to  stand  in  relation 
to  the  development  of  those  portions  of  the  trapezius  and 
sterno-mastoid  muscles  which  are  supplied  by  the  spinal 
accessory  nerve. 

It  is  believed  that  the  white  rami  communicantes  which 
pass  from  the  spinal  cord  to  the  thoracic  and  upper  lumbar 
sympathetic  ganglia  arise  from  cells  situated  in  the  dorso- 
lateral portions  of  the  ventral  horns,  and  it  is  noteworthy 
that  white  rami  are  wanting  in  the  region  in  which  the  spinal 


THE    CRANIAL    NERVES.  445 

accessory  nerve  occurs.  Since  this  nerve  represents  a  cranial 
lateral  motor  root  the  temptation  is  great  to  regard  the 
cranial  lateral  motor  roots  as  equivalent  to  the  white  rami 
of  the  cord,  and  this  temptation  is  intensified  when  it  is 
recalled  that  there  are  both  embryological  and  topographical 
reasons  for  regarding  the  branchiomeric  muscles,  to  which 
the  cranial  lateral  motor  nerves  are  supplied,  as  equivalent  to 
the  visceral  muscles  of  the  trunk.  But  in  view  of  the  fact 
that  a  sympathetic  neurone  is  always  interposed  between  a 
white  ramus  fiber  and  the  visceral  musculature,  while  the, 
lateral  motor  fibers  connect  directly  with  the  branchiomeric 
musculature,  it  seems  advisable  to  await  further  studies 
before  yielding  to  the  temptation. 

As  regards  the  actual  development  of  the  cranial  nerves, 
they  follow  the  general  law  which  obtains  for  the  spinal 
nerves,  the  motor  fibers  being  outgrowths  from  neuroblasts 
situated  in  the  walls  of  the  neural  tube,  while  the  sensory 
nerves  are  outgrowths  from  the  cells  of  ganglia  situated 
without  the  tube.  In  the  lower  vertebrates  a  series  of  gan- 
glia, known  as  the  suprabranchial  ganglia,  are  developed 
from  the  ectoderm  along  a  line  corresponding  with  the  level 
of  the  auditory  invagination,  while  on  a  line  corresponding 
with  the  upper  extremities  of  the  branchial  clefts  another 
series  occurs  which  has  been  termed  that  of  the  epihranchial 
ganglia,  and  with  both  of  these  sets  the  cranial  nerves  are 
in  connection.  In  the  mammalia  these  structures  have  not 
yet  been  sufficiently  studied,  but  from  the  general  relation- 
ship of  the  suprabranchial  ganglia  it  seems  probaole  that 
they  are  associated  with  the  lateral  line  nerves  and  are  con- 
sequently represented  in  the  mammalia  only  by  the  ganglia 
of  the  auditory  nerve. 

From  what  has  been  said  above  it  is  clear  that  the  usual 
arrangement  of  the  cranial  nerves  in  twelve  pairs  does  not  rep- 
resent their  true  relationships  with  one  another.  The  various 
pairs   are  serially  homologous  neither  with   one   another  nor 


44^  THE    SYMPATHETIC    SYSTEM. 

with  the  typical  spinal  nerves,  nor  can  they  be  regarded  as  rep- 
resenting twelve  cranial  segments.  Indeed,  it  would  seem 
that  comparatively  little  information  with  regard  to  the  number 
of  myotomic  segments  which  have  fused  together  to  form  the 
head  is  to  be  derived  from  the  cranial  nerves,  for  while  there 
are  only  four  of  these  nerves  which  are  associated  with  struc- 
tures equivalent  to  the  mesodermic  somites  of  the  trunk,  a 
much  greater  number  of  head  cavities  or  mesodermic  somites 
has  been  observed  in  the  cranial  region  of  the  embryos  of  the 
lower  vertebrates,  Dohrn,  for  instance,  having  found  nineteen 
and  Killian  eighteen  in  the  cranial  region  of  Toj'pcdo.  Furth- 
ermore, it  is  not  possible  to  say  at  present  whether  the  branchi- 
omeres  and  their  associated  nerves  correspond  with  one  or 
several  of  the  cranial  mesodermic  somites,  or  whether,  indeed, 
any  correspondence  whatever  exists. 

In  early  stages  of  development  a  series  of  constrictions  have 
been  observed  in  the  cranial  portion  of  the  neural  tube  and 
have  been  regarded  as  indicating  a  primitive  segmentation  of 
that  structure.  The  neuromercs,  as  the  intervals  between  suc- 
cessive constrictions  have  been  termed,  seem  to  correspond 
with  the  cranial  nerves  as  usually  recognized  and  hence  can- 
not be  regarded  as  primitive  segmental  structures.  They  are 
more  probably  secondary  and  due  to  the  arrangement  of  the 
neuroblasts  corresponding  to  the  various  nerves. 

The  Development  of  the  Sympathetic  Nervous  Sys- 
tem.— From  the  embryological  standpoint  the  distinction 
which  has  been  generally  recognized  between  the  sympa- 
thetic and  central  nervous  systems  does  not  exist,  the  former 
having  been  found  to  be  an  outgrowth  from  the  peripheral 
ganglia  of  the  latter.  This  mode  of  origin  has  been  observed 
with  especial  clearness  in  the  embryos  of  some  of  the  lower 
vertebrates,  in  which  masses  of  cells  have  been  seen  to  sepa- 
rate from  the  posterior  root  ganglia  to  form  the  ganglia  of 
the  ganglionated  cord  (Fig.  241).  In  the  mammalia,  in- 
cluding man,  the  relations  of  the  two  sets  of  ganglia  to  one 
another  is  by  no  means  so  apparent,  since  the  sympathetic 
cells,  instead  of  being  separated  from  the  posterior  root 
ganglion  en  masse,  migrate  from  it  singly  or  in  groups,  and 
are  therefore  less  readily  distinguishable  from  the  surround- 
ing mesodermal  tissues. 


THE    SYMPATHETIC    SYSTEM. 


44; 


^?:#^- 


■*^'  «#:::t^ 


^r-/  ^> 


.^cs> 


Fig.  241. — Transverse  Section  through  an  Embryo  Shark  (Scylliiiiii) 

OF    15    MM.,    SHOWING    THE    OrIGIN    OF    A    SYMPATHETIC    GaNGLION. 

Ch,  Notochord ;  E,  ectoderm ;   G,  posterior  root  ganglion ;   Gs,  sympa- 
thetic  ganglion;    M,   spinal    cord. —  {Onodi.) 


448 


THE    SYMPATHETIC    SYSTEM. 


To  understand  the  development  of  the  sympathetic  sys- 
tem it  must  be  remembered  that  it  consists  typically  of  three 
sets  of  ganglia.  One  of  these  is  constituted  by  the  ganglia 
of  the  ganglionated  cord  (Fig.  242,  GC),  the  second  is 
represented  by  the  ganglia  of  the  prsevertebral  plexuses 
(PVG),  such  as  the  cardiac,  solar,  hypogastric,  and  pelvic, 
v^hile  the  third  or  peripheral  set  (PG)  is  formed  by  the 
cells  which  occur  throughout  the  tissues  of  probably  most 
of  the  visceral  organs,  either  in  small  groups  or  scattered 
through  plexuses  such  as  the  Auerbach  and  Meissner  plex- 


FiG.  242.— Diagram  showing  the  Arrangement  of  the  Neurones 
OF   the   Sympathetic   System. 

The  fibers  from  the  posterior  root  ganglia  are  represented  by  the  broken 
black  lines ;  those  from  the  anterior  horn  cells  by  the  solid  black ; 
the  white  rami  by  red ;  and  the  sympathetic  neurones  by  blue. 
DR,  Dorsal  ramus  of  spinal  nerve ;  GC,  ganglionated  cord ;  GR,  gray 
ramus  communicans ;  PG,  peripheral  ganglion;  PVG,  prsevertebral 
ganglion ;  VR,  ventral  ramus  of  spinal  nerve ;  WR,  white  ramus 
communicans. —  {Adapted  from  Huber.) 


uses  of  the  intestine.  Each  cell  in  these  various  g'anglia 
stands  in  direct  contact  with  the  axis-cylinder  of  a  cell 
situated  in  the  central  nervous  system,  probably  in  the  lat- 
eral horn  of  the  spinal  cord  or  the  corresponding  region 
of  the  brain,  so  that  each  cell  forms  the  terminal  link  of  a 
chain  whose  first  link  is  a  neurone  belonging  to  the  central 
system    (Hul)er).     Throughout    the    thoracic    and    upper 


THE    SYMPATHETIC    SYSTEM.  449 

lumbar  regions  of  the  body  the  central  system  neurones 
form  distinct  cords  known  as  the  zuhite  rami  commiinic antes 
(Fig.  242,  WR),  which  pass  from  the  spinal  nerves  to  the 
adjacent  ganglia  of  the  ganglionated  cord,  some  of  them 
terminating  around  the  cells  of  these  ganglia,  others  pass- 
ing on  to  the  cells  of  the  prsevertebral  ganglia,  and  others 
to  those  of  the  peripheral  plexuses.  In  the  cervical,  lower 
lumbar  and  sacral  regions  white  rami  are  wanting,  the  cen- 
tral neurones  in  the  first-named  region  probably  making 
their  way  to  the  sympathetic  cells  by  way  of  the  upper 
thoracic  nerves,  while  in  the  lower  regions  they  may  pass 
down  the  ganglionated  cord  from  higher  regions  or  may 
join  the  prsevertebral  and  peripheral  ganglia  directly  with- 
out passing  through  the  proximal  ganglia.  In  addition  to 
these  white  rami,  what  are  known  as  gray  rami  also  extend 
between  the  proximal  gang'lia  and  the  spinal  nerves ;  these 
are  composed  of  fibers,  arising  from  sympathetic  cells,  which 
join  the  spinal  nerves  in  order  to  pass  with  them  to  their 
ultimate  distribution. 

The  brief  description  here  given  applies  especially  to  the 
sympathetic  system  of  the  neck  and  trunk.  Representa- 
tives of  the  system  are  also  found  in  the  head,  in  the  form 
of  a  series  of  g^ang'lia  connected  with  the  trigeminus  and 
facial  nerves  and  known  as  the  ciliary,  spheno-palatine,  otic, 
and  submaxillary  ganglia;  and,  as  will  be  seen  later,  there 
are  probably  some  sympathetic  cells  which  owe  their  origin 
to  the  root  ganglia  of  the  vagus  and  glossopharyngeal 
nerves.  There  is  nothing,  however,  in  the  head  region  cor- 
responding to  the  longitudinal  bundles  of  fibers  which  unite 
the  various  proximal  ganglia  of  the  trunk  to  form  the  gan- 
ghonated  cord. 

The  first  indications  of  the  sympathetic  system  are  to  be 
seen  in  a  human  embryo  of  about  7  mm.  As  the  spinal 
nerves  reach  the  level  of  the  dorsal  edge  of  the  body-cavity, 
39 


450 


THE    SYMPATHETIC    SYSTEM. 


they  branch,  one  of  the  branches  continuing  ventrahy  in 
the  body-wall,  while  the  other  (Fig.  243,  zm')  passes 
mesially  toward  the  aorta,  some  of  its  fibers  reaching  that 
structure,  while  others  bend  so  as  "to  assume  a  longitudinal 
direction.  These  mesial  branches  represent  the  white  rami 
communicantes,  but  as  yet  no  ganglion  cells  can  be  seen  in 


Fig.  243. — Transverse  Section  through  the  Spinal  Cord  of  an  Em- 
bryo OF  7  MM. 
c,  Notochord ;  g,  posterior  root  ganglion;  in,  spinal  cord;  .y,  sympathetic 

cell  migrating  from  the  posterior  root  ganglion;  zvr,  white  ramus. — 

iHis.) 

their  course.  The  cells  of  the  posterior  root  ganglia  have 
already,  for  the  most  part,  assumed  their  bipolar  form,  but 
among  them  there  may  still  be  found  a  number  of  cells  in 
the  neuroblast  condition,  and  these  (Fig.  243,  s) ,  wander- 
ing out  from  the  ganglia,  give  rise  to  a  column  of  cells 


•  THE    SYMPATHETIC    SYSTEM.  451 

standing-  in  relation  to  the  white  rami.  At  first  there  is 
no  indication  of  a  segmental  arrangement  of  the  cells  of 
the  column  (Fig.  244),  but  at  about  the  seventh  week  such 
an  arrangement  makes  its  appearance  in  the  cervical  region, 
and  later,  extends  posteriorly,  until  the  column  assumes  the 
form  of  the  ganglionated  cord. 

Before,  however,  the  segmentation  becomes  marked, 
thickenings  appear  at  certain  regions  of  the  cell  column, 
and  from  these,  bundles  of  fibers  may  be  seen  extending 
ventrally  toward  the  viscera.  The  thickenings  represent 
certain  of  the  prsevertebral  ganglia,  and  later  cells  wander 
out  from  them  and  take  a  position  in  front  of  the  aorta. 
In  an  embryo  of  10.2  mm.  two  ganglionic  masses  (Fig. 
244,  pc)  occur  in  the  vicinity  of  the  origin  of  the  omphalo- 
mesenteric artery  {am),  one  lying  above  and  the  other 
below  that  vessel ;  these  masses  represent  the  ganglia  of 
the  coeliac  plexus  and  have  separated  somewhat  from  the 
ganglionated  cord,  the  fiber  bundles  which  unite  the  upper 
mass  with  the  cord  representing  the  greater  and  lesser 
splanchnic  nerves  {sp),  while  that  connected  with  the  lower 
mass  represents  the  connection  of  the  cord  w4th  the  superior 
mesenteric  ganglion.  Lower  down,  in  the  neighborhood 
of  the  umbilical  arteries,  is  another  enlargement  of  the  cord 
{hg),  which  probably  represents  the  inferior  mesenteric 
and  hypogastric  ganglia  which  have  not  yet  separated  from 
the  cell  column. 

In  the  cervical  region  a  similar  origin  of  the  ganglion 
cells  of  the  cardiac  plexus  from  the  cell  column  seems  to 
obtain.  In  embryos  of  about  7  mm.  fibers  may  be  seen 
extending  from  the  column  toward  the  heart,  and,  entering 
into  close  relationship  with  descending  branches  from  the 
vagus,  they  form  a  plexus,  the  ganglia  of  which  are  com- 
posed of  cells  which  have  wandered  from  the  cell  column. 


452 


THE    SYMPATHETIC    SYSTEM, 


The  elongated  courses  of  the  cardiac  sympathetic  and 
splanchnic  nerves  in  the  adult  receive  an  explanation  from  the 
recession  of  the  heart  and  diaphragm  (see  pp.  252  and  342), 
the  latter  process  forcing  downward  the  coeliac  plexus,  which 
originally  occupied  a  position  opposite  the  region  of  the  gan- 
glionated  cord  from  which  the  splanchnic  nerves  arise. 


Fig.  244. — Reconstruction  of  the  Sympathetic  System  of  an  Em- 
bryo OF    10.2    MM. 

am,  Omphalo-mesenteric  artery;  ao,  aorta;  au,  umbilical  artery;  bg, 
ganglionic  mass  representing  the  pelvic  plexus;  d,  intestine;  oe, 
ffisophagus;  pc,  ganglia  of  the  cceliac  plexus;  ph,  pharynx;  rv, 
right  vagus  nerve;  sp,  splanchnic  nerves;  sy,  ganglionated  cord; 
t,  trachea;*,  peripheral  sympathetic  ganglia  in  the  walls  of  the 
stomach. —  (^His,  Jr.) 


THE   SYMPATHETIC    SYSTEM.  453 

The  cells  which  occur  in  the  peripheral  plexuses  have, 
in  a  similar  manner,  wandered  out  from  their  original  posi- 
tion in  the  cell  column.  In  lo  mm.  embryos  groups  of 
such  cells  have  been  observed  both  in  the  lesser  and  greater 
curvatures  of  the  stomach  (Fig.  244*),  where  they  become 
connected  with  a  plexus  formed  by  fibers  from  the  vagus 
nerves  (rv).  The  wandering  of  sympathetic  cells  into  the 
walls  of  the  intestine  has  also  been  observed,  and  they  form 
at  first  a  single  layer  in  the  mesoderm  of  the  intestinal  wall, 
only  later,  on  the  differentiation  of  the  muscle  layers,  becom- 
ing separated  into  the  two  layers  which  constitute  the  plex- 
uses of  Auerbach  and  Meissner.  Similarly  cells  reach  the 
heart  by  wandering  in  some  cases  along  fibers  of  the  vagus, 
although  they  really  come  from  the  cervical  region  of  the 
g-anglionated  cord,  and,  having  in  their  wandering  met  with 
fibers  of  the  vagus,  make  use  of  them  as  paths  by  which  they 
may  reach  their  destination. 

As  regards  the  cephalic  sympathetic  ganglia,  the  observa- 
tions of  Remak  on  the  chick  and  Kolliker  on  the  rabbit  show 
that  the  ciliary,  sphenopalatine,  and  otic  ganglia  arise  by 
the  separation  of  cells  from  the  semilunar  (Gasserian)  gan- 
glion, and  from  their  adult  relations  it  may  be  supposed  that 
the  cells  of  the  submaxillary  and  sublingual  ganglia  have 
similarly  arisen  from  the  geniculate  ganglion  of  the  facial 
nerve.  Evidence  has  also  been  obtained  from  human  em- 
bryos that  sympathetic  cells  are  derived  from  the  ganglia 
of  the  vagus  and  glossopharyngeal  nerves,  but,  instead  of 
forming  distinct  ganglia  in  the  adult,  these,  in  all  proba- 
bility, associate  themselves  with  the  first  cervical  ganglia  of 
the  ganglionated  cord. 


454  LITERATURE. 

LITERATURE. 

S.  R.  Cajal:  "Die  histogenetische  Beweise  der  Neuronentheorie  von 

His  und  Forel,"  Anat.  Anzeiger,  xxx,  1907. 
K.  Goldstein  :  "  Die  erste  Entwickelung  der  grossen  Hirncommissuren 

und  die  '  Verwachsung '  von  Thalamus  und  Striatum,"  Archiv  fiir 

Anat.  und  Physiol,  Anat.  Abth.,  1903. 
G.    Groexberg  :    "  Die    Ontogenese   einer   niederen    Saugergehirns    nach 

Untersucliungen    an    Erinaceus    europaeus,"   Zoolog.   Jahrb.    Abth. 

f.  Anat.  und  Ontogen.,  xv,  1901. 
R.  G.  Harrison  :  "  Further  Experiments  on  the  Development  of  Peri- 
pheral Nerves,"  Anier.  Joiirn.  of  Anat.,  v,  1906. 
W.   His  :    "  Zur   Geschichte  des  menschlichen  Riickenmarkes   und   der 

Nervenwurzeln,"     Abhandl.     der    konigl.     Sachsischen    Gesellsch., 

Math.-Physik.  Classe,  xiii,  1886. 
W.  His  :  "  Zur  Geschichte  des  Gehirns  sowie  der  centralen  und  peripher- 

ischen   Nervenbahnen   beim   menschlichen   Embryo,"   Abhandl.   der 

konigl.  Sachsischen  Gesellscli.,  MatJi.-Physik.  Classe,  xiv,  1888. 
W.  His  :  "  Die  Formentwickelung  des  menschlichen  Vorderhirns  vom 

Ende  des   ersten  bis   zum   Beginn   des   dritten   Monats,"  Abhandl. 

der  konigl.  Sachsischen  Gesellsch.,  Math.-Physik.  Classe,  xv,  1S89. 
W.    His  :    "  Histogenese    und    Zusammenhang    der    Nervenelemente," 

Archiv  fiir  Anat.  und  Physiol.,  Anat.  Abth.,  Supplement,   1890. 
W.   His :   "  Die  Entwickelung  des  menschlichen   Gehirns   wahrend  der 

ersten   Monate,"   Leipzig,   1904. 
W.  His,  Jr.  :  "  Die  Entwickelung  des  Herznervensystems  bei  Wirbel- 

thieren,"  Abhandl.  der  konigl.  Sachsischen  Gesellsch.,  Math.-Physik. 

Classe,  xviii,  1893. 
W.   His,  Jr.  :    "  Ueber  die   Entwickelung  des   Bauchsympathicus  beim 

HiJhnchen  und  Menschen,"  Archiv  fiir  Anat.  und  Physiol,  Anat. 

Abth.,  Supplement,  1897. 
C.  J.  Herrick  :  "  The  Cranial  and  First  Spinal  Nerves  of  Mcnidia :  A 

Contribution   upon   the  Nerve   Components   of  the   Bony  Fishes," 

Journ.  of  Comp.  Neurol,  ix,  1899. 
C.  J.  Herrick  :  "  The  Cranial  Nerves  and  Cutaneous  Sense-organs  of 

the  North  American   Siluroid  Fishes,"  Journ.   of  Comp.   Neurol, 

XX,  1901. 
G.  C.  HuBER :  "  Four  Lectures  on  the  Sympathetic  Nervous  System," 

Journ.  of  Comp.  Neurol,  vii,  1897. 
M.  VON  Lenhossek  :  "  Die  Entwickelung  der  Ganglienanlagen  bei  dem 

menschlichen  Embryo,"  Archiv  fiir  Anat.  und  Physiol,  Anat.  Abth., 

1891. 
F.  Marchand:  "Ueber  die  Entwickelung  des  Balkens  im  menschlichen 

Gehirn,"  Archiv  fiir  mikrosk.  Anat.,  xxxvii,  1891. 


LITERATURE.  455 

V.  VON  MiHALKOVicz :  "  Entwickelungsgeschichte  des  Gehirns,"  Leipzig, 

1877. 
A.  D.  Onodi  :  "  Ueber  die  Entwickelung  des  sympathischen  Nervensys- 

tems/'  Archiv  fiir  mikrosk.  Anat.,  xxvii,  1886. 
G.  Retzius  :  "  Das  Menschenhirn,"  Stockholm,  1896. 
A.    ScHAPER :    "  Die    friihesten    Differenzirungsvorgange    im    Central- 

nerven-system,"  Archiv  fiir  Entzvickhmgsmechanik,  v,    1897. 
G.  L.  Streeter:  "The  Development  of  the  Cranial  and  Spinal  Nerves 

in   the   Occipital   Region   of  the   Human   Embryo,"  Amer.   Joiirn. 

Anat.,  IV,  1904. 
O.  S.  Strong  :  "  The  Cranial  Nerves  of  Amphibia,"  Journal  of  Mor- 

phoL,  X,  1895. 
R.  Wlassak  :  "  Die  Herkunft  des  Myelins,"  Archiv  fiir  Entwicklungs- 

mechanik,  vi,  1898. 
E.  Zuckerkandl:  "  Znr  Entwickelung  des  Balkens  und  des  Gewolbes," 

Sitsungsbcr.  kais.  Acad.  IVissensch.  Wien.;  Math.-Natur'W.     Classe, 

ex,  1901. 


CHAPTER  XVI. 

THE   DEVELOPMENT   OF  THE  ORGANS   OF 
SPECIAL  SENSE. 

Like  the  cells  of  the  central  nervous  system,  the  sensory 
cells  are  all  of  ectodermal  origin,  and  in  lower  animals,, 
such  as  the  earthworm,  for  instance,  they  retain  their  orig- 
inal position  in  the  ectodermal  epithelium  throughout  life. 
In  the  vertebrates,  however,  the  majority  of  the  sensory 
cells  relinc[uish  their  superficial  position  and  sink  more  or 
less  deeply  into  the  subjacent  tissues,  being  represented  by 
the  posterior  root  ganglion  cells  and  by  the  sensory  cells  of 
the  special  sense-organs,  and  it  is  only  in  the  olfactory  organ 
that  the  original  condition  is  retained.  Those  cells  which 
have  withdrawn  from  the  surface  receive  stimuli  only 
through  an  overlying'  cell  or  cells,  and  in  certain  cases  these 
transmitting  cells  are  not  specially  differentiated,  the  ter- 
minal branches  of  the  sensory  dendrites  ending  among  ordi- 
nary epithelial  cells  or  in  such  structures  as  the  Pacinian 
bodies  or  the  end-bulbs  of  Krause  situated  beneath  undiffer- 
entiated epithelium.  In  other  cases,  however,  certain  spe- 
cially modified  superficial  cells  serve  to  transmit  the  stimuli 
to  the  peripheral  sensory  neurones,  forming  such  structures 
as  the  hair-cells  of  the  auditory  epithelium  or  of  the  taste- 
buds. 

Thus  three  degrees  of  differentiation  of  the  special  sen- 
sory cells  may  be  recognized  and  a  classification  of  the 
sense-organs  may  be  made  upon  this  basis.  One  organ, 
however,  the  eye,  cannot  be  brought  into  such  a  classifica- 
tion,  since  its  sensory  cells  present  certain  developmental 

456 


THE    OLFACTORY    ORGAN.  45/ 

peculiarities  which  distinguish  them  from  those  of  all  other 
sense-organs.  Embryologically  the  retina  is  a  portion  of 
the  central  nervous  system  and  not  a  peripheral  organ,  and 
hence  it  will  be  convenient  to  arrange  the  other  sense-organs 
according  to  the  classification  indicated  and  to  discuss  the 
history  of  the  eye  at  the  close  of  the  chapter. 

The  Development  of  the  Olfactory  Organ. — The  gen- 
eral development  of  the  nasal  fossa,  the  epithelium  of  which 
contains  the  olfactory  sense  cells,  has  already  been  described 
(pp.  8i  and  89),  as  has  also  the  development  of  the  olfac- 
tory lobes  of  the  brain  (p.  433),  and  there  remains  for  con- 
sideration here  merely  the  formation  of  the  olfactory  nerve 
and  the  development  of  the  rudimentary  organ  of  Jacobson. 

The  Olfactory  Nerve. — Very  diverse  results  have  been 
obtained  by  various  observers  of  the  development  of  the 
olfactory  nerve,  it  having  been  held  at  different  times  that 
it  was  formed  by  the  outgrowth  of  fibers  from  the  olfac- 
tory lobes  (Marshall),  from  fibers  which  arise  partly  from 
the  olfactory  lobes  and  partly  from  the  olfactory  epithelium 
(Beard),  from  the  cells  of  an  olfactory  ganglion  originally 
derived  from  the  olfactory  epithelium  but  later  separating 
from  it  (His),  and,  finally,  that  it  was  composed  of  the  pro- 
longations of  certain  cells  situated  and,  for  the  most  part  at 
least,  remaining  permanently  in  the  olfactory  epithelium 
(Disse).  The  most  recent  observations  on  the  structure  of 
the  olfactory  epithelium  and  nerve  indicate  a  greater  amount 
of  probability  in  the  last  result  than  in  the  others,  and  the 
description  which  follows  will  be  based  upon  the  observa- 
tions of  His,  modified  in  conformity  with  the  results  ob- 
tained by  Disse  from  chick  embryos. 

In  human  embryos  of  the  fourth  week  the  cells  lining 

the  upper  part  of  the  olfactory  pits  show  a  distinction  into 

ordinary  epithelial  and  sensory  cells,  the  latter,  when  fully 

formed,  being  elongated  cells  prolonged  peripherally  into  a 
40 


458 


THE    OLFACTORY    ORGAN. 


short  but  narrow  process  which  reaches  the  surface  of  the 
epithehum  and  proximally  gives  rise  to  an  axis-cyhnder 
process  which  extends  up  toward  and  penetrates  the  tip  of 
the  olfactory  lobe  to  come  into  contact  with  the  dendrites 
of  the  first  central  neurones  of  the  olfactory  tract    (Fig. 


Fig.   245. — Diagram    Illustrating   the  Relations   of  the   Fibers   of 

THE   Olfactory    Nerve. 
Ep,  Epithelium  of  the  olfactory  pit;  C,  cribiform  plate  of  the  ethmoid, 

G,  glomerulus  of  the  olfactory  bulb;   M,   mitral  cell. —  {Van  Ge- 

huchten.) 

245).  These  cells  constitute  a  neuro-epithelium  and  in  later 
stages  of  development  retain  their  epithelial  position  for  the 
most  part,  a  few  of  them,  however,  withdrawing  into  the 
subjacent  mesenchyme  and  becoming  bipolar,  their  periph- 


THE    OLFACTORY    ORGAN.  459 

eral  prolong-ations  ending  freely  among  the  cells  of  the  ol- 
factory epithelium.  These  bipolar  cells  resemble  closely  in 
form  and  relations  the  cells  of  the  embryonic  posterior  root 
ganglia,  and  thus  form  an  interesting  transition  between 
these  and  the  neuro-epithelial  cells. 

The  Organ  of  Jacohson. — In  embryos  of  three  or  four 
months  a  small  pouch-like  invagination  of  the  epithelium 
covering  the  lower  anterior  portion  of  the  median  septum 
of  the  nose  can  readily  be  seen.  This  becomes  converted 
into  a  slender  pouch,  3  to  5  mm.  long,  ending  blindly  at  its 
posterior  extremity  and  opening  at  its  other  end  into  the 
nasal  cavity.  Its  lining  epithelium  resembles  that  of  the 
respiratory  portion  of  the  nasal  cavity,  and  there  is  devel- 
oped in  the  connective  tissue  beneath  its  floor  a  slender 
plate  of  cartilage,  distinct  from  that  forming  the  septum  of 
the  nose. 

This  organ,  which  may  apparently  undergo  degeneration 
in  the  adult,  and  in  some  cases  completely  disappears,  ap- 
pears to  be  the  representative  of  what  is  known  as  Jacobson's 
organ,  a  structure  which  reaches  a  much  more  extensive 
degree  of  development  in  many  of  the  lower  mammals,  and 
in  these  contains  in  its  epithelium  sensory  cells  whose  axis- 
cylinder  processes  pass  with  those  of  the  olfactory  sense  cells 
to  the  olfactory  bulbs.  In  man,  however,  it  seems  to  be  a 
rudimentary  organ,  and  no  satisfactory  explanation  of  its 
function  has  as  yet  been  advanced. 

The  olfactory  neuro-epithelium,  considered  from  a  com- 
parative standpoint,  seems  to  ha\'e  been  derived  from  the 
system  of  lateral  line  organs  so  highly  developed  in  the  lower 
vertebrates  (Kupfifer).  In  higher  forms  the  system,  which 
is  cutaneous  in  character,  has  disappeared  except  in  two 
regions  where  it  has  become  highly  specialized.  In  one  of 
these  regions  it  has  given  rise  to  the  olfactory  sense  cells  and 
in  the  other  to  the  similar  cells  of  the  auditory  apparatus. 


460 


THE    ORGANS    OF    TASTE. 


The  Organs  of  Touch  and  Taste. — Nothing  is  yet 
known  concerning  the  development  of  the  various  forms  of 
tactile  organs,  which  belong  to  the  second  class  of  sensory 
organs  described  above. 

The  Organs  of  Taste. — The  remaining  organs  of  special 
sense  belong  to  the  third  class,  and  of  these  the  organs  of 
taste  present  in  many  respects  the  simplest  condition.  They 
are  developed  principally  in  connection  with  the  vallate  and 
foliate  papihse  of  the  tongue,  and  of  the  former  one  of  the 
earliest  observed  stages  has  been  found  in  embryos  of  9 
cm.  in  the  form  of  two  ridges  of  epidermis,  lying  toward 


A  4P^'    -   ^i 

' B    ' C 

Fig.   246. — Diagrams  Representing  the  Development  of  a  Vallate 

Papilla. 
a,  Valley  surrounding  the  papilla;  b,  von  Ebner's  gland. —  (Graberg.) 


the  back  part  of  the  tongue  and  inclined  to  one  another  in 
such  a  manner  as  to  form  a  V  with  the  apex  directed  back- 
ward. From  these  ridges  solid  downgrowths  of  epidermis 
into  the  subjacent  tissue  occur,  each  downgrowth  having  the 
form  of  a  hollow  truncated  cone  with  its  basal  edge  con- 
tinuous with  the  superficial  epidermis  (Fig.  246,  A).  In 
later  stages  lateral  outgrowths  develop  from  the  deeper 
edges  of  the  cone,  and  about  the  same  time  clefts  appear  in 
the  substance  of  the  original  downgrowths  (Fig.  246,  B) 
and,  uniting  together,  finally  open  to  the  surface,  forming 
a  trench  surrounding  a  papilla  (Fig.  246,  C).  The  lateral 
outgrowths,  which  are  at  first  solid,  also  undergo  an  axial 
degeneration  and  become  converted  into  the  glands  of  Ebner 
(h),  which  open  into  the  trench  near  its  floor.     The  various 


THE    INTERNAL  EAR.  40 1 

papillae  which  occur  in  the  adult  do  not  develop  simulta- 
neously, but  their  number  increases  with  the  age  of  the 
fetus,  and  there  is,  moreover,  considerable  variation  in  the 
time  of  their  development. 

The  taste-buds  are  formed  by  a  differentiation  of  the 
epithelium  which  covers  the  papilte,  and  this  differentiation 
appears  to  stand  in  intimate  relation  with  the  penetration  of 
fibers  of  the  glossopharyngeal  nerve  into  the  papillae.  The 
buds  form  at  various  places  upon  the  papillae,  and  at  one 
period  are  especially  abundant  upon  their  free  surfaces,  but 
in  the  later  weeks  of  intrauterine  life  these  surface  buds 
undergo  degeneration  and  only  those  upon  the  sides  of  the 
trench  persist,  as  a  rule. 

The  foliate  papillae  do  not  seem  to  be  developed  until  some 
time  after  the  circumvallate,  being  entirely  wanting  in  em- 
bryos of  four  and  a  half  and  five  months,  although  plainly 
recognizable  at  the  seventh  month. 

The  Development  of  the  Ear. — It  is  customary  to  de- 
scribe the  mammalian  ear  as  consistingof  three  parts,  known 
as  the  inner,  middle,  and  outer  ears,  and  this  division  is,  to 
a  certain  extent  at  least,  confirmed  by  the  embryonic  devel- 
opment. The  inner  ear,  which  is  the  sensory  portion  proper, 
is  an  ectodermal  structure,  which  secondarily  becomes  deeply 
seated  in  the  mesodermal  tissue  of  the  head,  while  the  middle 
and  outer  ears,  which  provide  the  apparatus  necessary  for 
the  conduction  of  the  sound-waves  to  the  inner  ear,  are  modi- 
fied portions  of  the  anterior  branchial  arches.  It  will  be 
convenient,  accordingly,  in  the  description  of  the  ear,  to 
accept  the  usually  recognized  divisions  and  to  consider  first 
of  all  the  development  of  the  inner  ear,  or,  as  it  is  better 
termed,  the  otocyst. 

The  Development  of  the  Otocyst. — In  an  embryo  of  2.4 
mm.  a  pair  of  pits  occur  upon  the  surface  of  the  body  about 
opposite  the  middle  portion  of  the  hind-brain  (Fig.  247,  A), 


462 


THE    INTERNAL    EAR. 


The  -ectoderm  lining  the  pits  is  somewhat  thicker  than  is  the 
neighboring  ectoderm  of  the  surface  of  the  body,  and,  from 
analogy  with  what  occnrs  in  other  vertebrates,  it  seems  prob- 
able that  the  pits  are  formed  by  the  invagination  of  localized 
thickenings  of  the  ectoderm.  The  mouth  of  each  pit  grad- 
ually becomes  smaller,  until  finally  the  invagination  is  con- 
verted into  a  closed  sac  (Fig.  247,  B),  which  separates  from 
the  surface  ectoderm  and  becomes  enclosed  within  the  sub- 
jacent mesoderm.  This  sac  is  the  otocyst,  and  in  the  stage 
just  described,  found  in  embryos  of  4  mm.,  it  has  an  oval 
or  more  or  less  spherical  form.  Soon,  however,  in  embryos 
of  6.9  mm.,  a  prolongation  arises  from  its  dorsal  portion 


Fig.  247.— Transverse  Section   Passing  through  the  Otocyst   (ot) 
OF  Embryos  of   (A)   2.4  mm.  and   (B)   4  UM.—  (His.) 

and  the  sac  assumes  the  form  shown  in  Fig.  248,  A;  this 
prolongation,  which  is  held  by  some  authors  to  be  the  re- 
mains of  the  stalk  which  originally  connected  the  otocyst 
sac  with  the  surface  ectoderm,  represents  the  diictns  endo- 
lymphaticus,  and,  increasing  in  length,  it  soon  becomes  a 
strong  club-shaped  process,  projecting  considerably  beyond 
the  remaining  portions  of  the  otocyst  (Fig.  248,  B).  In 
embryos  of  about  10.2  mm.  the  sac  begins  to  show  certain 
otlier  irregularities  of  shape  (Fig.  248,  'B,sc).  Thus,  about 
opposite  the  point  of  origin  of  the  ductus  endolymphaticus 
three  folds  make  their  appearance,  representing  the  semi- 
circular  (htcfs^  and  as  they  increase  in  size  the  opposite  walls 


THE    INTERNAL  EAR. 


463 


of  the  central  portion  of  each  fold  come  together,  fuse,  and 
finally  become  absorbed,  leaving  the  free  edge  of  the  fold 
as  a  crescentic  canal,  at  one  end  of  which  an  enlargement 
appears  to  form  the  ampulla.  The  transformation  of  the 
folds  into  canals  takes  place  somewhat  earlier  in  the  cases 
of  the  two  vertical  than  in  that  of  the  horizontal  duct,  as 


Fig.  248. — Reconstruction  of  the  Otocysts  of  Embryo  of   (A)   6.9 

MM.     AND     (B)      10.2     MM. 

dc,  Endolymphatic  duct;  gc,  ganglion  cochleare;  gg,  ganglion  genicula- 
tum ;  gv,  ganglion  vestibulare ;  sc,  lateral  semicircular  duct. — 
(His,  Jr.) 

may  be  seen  from  Fig.  249,  which  represents  the  condition 
occurring  in  an  embryo  of  13.5  mm. 

A  short  distance  below  the  level  at  which  the  canals  com- 
municate with  the  remaining  portion  of  the  otocyst  a  con- 
striction appears,  indicating  a  separation  of  the  otocyst  into 
a  more  dorsal  portion  and  a  more  ventral  one.     Later,  the 


464 


THE    INTERNAL    EAR. 


latter  begins  to  be  prolong-ed  into  a  flattened  canal  which, 
as  it  elongates,  becomes  coiled  upon  itself  and  also  becomes 
separated  by  a  constriction  from  the  remaining  portion  of 
the  otocyst  (Fig.  250).  This  canal  is  the  ductus  cochlearis 
(scala  media  of  the  cochlea),  and  the  remaining  portion  of 

the  otocyst  subsequently  be- 
comes divided  by  a  constriction 
into  the  utyicuhis,  with  which 
the  semicircular  ducts  are  con- 
nected, and  the  sacculus.  The 
constriction  which  separates 
the  cochlear  duct  from  tlie 
sacculus  becomes  the  ductus 
rcuiiicus.  while  that  between 
the  utricuhis  and  sacculus  is 
converted  into  a  narrow  can.al 
with  which  the  ductus  endo- 
lymphaticus  connects,  and 
hence  it  is  that,  in  the  adult, 
the  connection  between  these 
two  portions  of  the  otocyst 
seems  to  be  formed  by  the 
ductus  dividing  proximally  into 
two  limbs,  one  of  which  is 
connected  with  the  utricle  and 
the  other  with  the  saccule. 

When  first  observed  in  the 
human    embryo    the    auditory 


Fig.  249. — Reconstruction  of 
THE  Otocyst  of  an  Embryo 

OF    13.5    MM. 

CO,  Cochlea;  de,  endolymphatic 
duct ;  sc.  semicircular  duct 
—  (His,  Jr.) 


ganglion   is   closely   associated 
geniculate    ganglion 


with    the 

of  the  seventh  nerve  (Fig.  248,  B),  the  two,  usually  spoken 
of  as  the  acustico-facialis  ganglion,  forming  a  mass  of  cells 
lying  in  close  contact  with  the  anterior  wall  of  the  otocyst. 
The  origin  of  the  ganglionic  mass  has  not  yet  been  traced 


THE    INTERNAL  EAR. 


465 


in  the  mammalia,  but  it  has  been  observed  that  in  cow  em- 
bryos the  g-eniculate  ganghon  is  connected  with  the  ectoderm 
at  the  dorsal  end  of  the  first  branchial  cleft  (Froriep),  and 
it  may  perhaps  be  regarded  as  one  of  the  epibranchial  gan- 
glia (see  p.  445),  and  in  the  lower  vertebrates  a  union  of 
the  ganglion  with  a  suprabranchial  ganglion  has  been  ob- 
served  (Kupffer),  this  union  indicating  the  origin  of  the 


Sdl. 


Fig.  250. — Reconstruction  of  the  Otocyst  of  an  Embryo  of  20  mm., 

front  view. 
cc,    common    limb    of    superior   and   posterior    semicircular    ducts ;    eg, 

cochlear  ganglion ;  co,  cochlea ;  de,  endolymphatic  duct ;  s,  sacculus ; 

sdl,  sdp.  and  sds,  lateral,  posterior  and  superior  semicircular  ducts; 

u,  utriculus;  vg,  vestibular  ganglion. —  (Streeter.) 

auditory  ganglion  from  one  or  more  of  the  ganglia  of  the 
lateral  line  system. 

At  an  early  stage  in  the  human  embryo  the  auditory 
ganglion  shows  indications  of  a  division  into  two  portions, 
a  more  dorsal  one,  which  represents  the  future  ganglion 
vestihiilare,  and  a  ventral  one,  the  ganglion  cochleare.  The 
ganglion  cells  become  bipolar,  in  which  condition  they  re- 


466 


THE    INTERNAL    EAR. 


main  throughout  Hfe,  never  reaching  the  T  -shai:)ed  condi- 
tion found  in  most  of  the  other  peripheral  cerebro-spinal 
gangha.  One  of  the  prolongations  of  each  cell  is  directed 
centrally  to  form  a  fiber  of  the  auditory  nerve,  while  the 
other  penetrates  the  wall  of  the  otocyst  to  enter  into  rela- 


FiG.  251. — The  Right  Internal  Ear  of  an  Embryo  of  Six  Months. 

ca,  ce,  and  cp,  Superior,  lateral,  and  posterior  semicircular  ducts ;  cr, 
crista  acustica;  de,  endolymphatic  duct;  Is,  spiral  ligament;  mb, 
basilar  membrane ;  ins  and  mu,  macula  acustica  sacculi  and  utri- 
culi;  rb,  basilar  branches  of  the  cochlear  nerve. —  (Retsius.) 

tions  witli  certain  specially  modified  cells  which  differentiate 
from  its  lining  epithelium. 

In  the  earliest  stages  the  ectodermal  lining  of  the  otocyst 
is  formed  of  similar  columnar  cells,  but  later  over  the  greater 


THE    INTERNAL  EAR.  4^7 

part  of  the  surface  the  cehs  flatten  down,  only  a  few,  aggre- 
gated together  to  form  patches,  retaining  the  high  columnar 
form  and  developing  hair-like  processes  upon  their  free  sur- 
faces. These  are  the  sensory  cells  of  the  ear.  In  the 
human  ear  there  are  in  all  six  patches  of  these  sensory  cells, 
an  elongated  patch  {crista  ampullar  is)  in  the  ampulla  of 
each  semicircular  canal  (Fig.  251,  cr) ,  a  round  patch  {ma- 


fy^v- 


''; 


i3'i   S  S 


v:iT-''^'V 


^^  'n;.> 


>       ^7 


n 


%1 


I 


Fig.   252. — Section  of  the  Cochlear  Duct  of  a  Rabbit   Embryo  of 

55    MM. 

a,  Mesenchyme;   b  to  c,  epithelium  of  cochlear  duct;   M.t,  membrana 
tectoria;  V.s.p,  vein;  i  to  7,  spiral  organ  of  Corti. —  (Baginsky.) 

Cilia  acustica,  mu)  in  the  utriculus  and  another  (ms)  in  the 
sacculus,  and,  finally,  an  elongated  patch  which  extends  the 
entire  length  of  the  scala  media  of  the  cochlea  and  forms 
the  sensory  cells  of  the  spiral  organ  of  Corti.  The  cells  of 
this  last  patch  are  connected  with  the  fibers  from  the  coch- 
lear ganglion,  while  those  of  the  vestibular  ganglion  pass 
to  the  cristse  and  maculae. 


468  THE    INTERNAL    EAR. 

In  connection  with  the  spiral  organ  certain  adjacent  cells 
also  retain  their  columnar  form  and  undergo  various  modi- 
fications, giving  rise  to  a  rather  complicated  structure  whose 
development  has  been  traced  in  the  rabbit.  Along  the  whole 
length  of  the  cochlear  duct  the  cells  resting  upon  that  half 
of  the  basilar  membrane  which  is  nearest  the  axis  of  the 
cochlea,  and  may  be  termed  the  inner  half,  retain  their 
columnar  shape,  forming  two  ridges  projecting  slightly  into 
the  cavity  of  the  scala  (Fig.  252).  The  cells  of  the  inner 
ridge,  much  the  larger  of  the  two,  give  rise  to  the  nieni- 
hrana  tcctoria,  either  as  a  cuticular  secretion  or  by  the  arti- 
ficial adhesion  of  long  hair-like  processes  which  project  from 
their  free  surfaces  (Ayers).  The  cells  of  the  outer  ridge 
are  arranged  in  six  longitudinal  rows  (Fig.  252,  1-6)  ;  those 
of  the  innermost  row  ( i )  develop  hairs  upon  their  free  sur- 
faces and  form  the  inner  hair  cells,  those  of  the  next  two 
rows  (2  and  3)  gradually  become  transformed  on  their 
adjacent  surfaces  into  chitinous  substance  and  form  the  rods 
of  Corti,  while  the  three  outer  rows  (4  to  6)  develop  into 
the  outer  hair  cells.  It  is  in  connection  with  the  hair  cells 
that  the  peripheral  prolongations  of  the  cells  of  the  cochlear 
ganglion  terminate,  and  since  these  hair  cells  are  arranged 
in  rows  extending  the  entire  length  of  the  cochlear  duct,  the 
ganglion  also  is  drawn  out  into  a  spiral  following  the  coils 
of  the  cochlea,  and  hence  is  sometimes  termed  the  spiral 
ganglion. 

While  the  various  changes  described  above  have  been 
taking  place  in  the  otocyst,  the  mesoderm  surrounding  it 
has  also  been  undergoing  development.  At  first  this  tissue 
is  Cjuite  uniform  in  character,  but  later  the  cells  immediately 
surrounding  the  otocyst  condense  to  give  rise  to  a  fibrous 
layer  (Fig.  253,  ep)  while  more  peripherally  they  become 
more  loosely  arranged  and  form  a  somewhat  gelatinous 
layer  (s),  and  still  more  peripherally  a  second  fibrous  layer 


THE    INTERNAL  EAR.  469 

is  differentiated  and  the  remainder  of  the  tissue  assumes  a 
character  which  indicates  an  approaching  conversion  into 
cartilage.  The  further  history  of  these  various  layers  is 
as  follows :  The  inner  fibrous  layer  gives  rise  to  the  con- 
nective-tissue wall  which 
supports  the  ectodermal 
lining     of     the     various 

portions   of   the   otocyst;     P -■- — 

the   gelatinous   layer   un-     ^  ~^         .  ',\ 

dergoes  a  degeneration  to  ^  \ 

form   a   lymph-like   fluid     ^^  o.^    ■^•- 

known  as  the  perilymph,  ^  ^     '^ 

the  space  occupied  by  the 

fluid   being   the   perilym-      ^  ^  ^ 

.  riG.       253.^ — Transverse       Section 

phatlC     space ;     the     outer  through     a     Semicircular     Duct 

fibrous  layer  becomes  peri-         o^  ^  Rabbit  Embryo  of  Twenty- 
four  Days. 
chondrium  and  later  pen-      ,^  v^,\o\:xz  cartilage;  ep,  fibrous  mem- 
OSteum;  and  the  procarti-  brane    beneath    the    epithelium    of 

1  ,  .         ^   . r~  the    canal;    p,    perichondrium;    s, 

lage  undergoes  chondrifi-         sp^^gy  tissue.-(Fo7t  Kdlliker.) 

cation  and  later  ossifies  to 

form  the  petrous  portion  of  the  temporal  bone. 

The  gelatinous  layer  completely  surrounds  most  of  the 
otocyst  structures,  which  thus  come  to  lie  free  in  the  peri- 
lymphatic space,  but  in  the  cochlear  region  the  conditions 
are  somewhat  different.  In  this  region  the  gelatinous 
layer  is  interrupted  along  two  lines,  an  outer  broad  one 
where  the  connective-tissue  wall  of  the  cochlear  duct  is 
directly  continuous  with  the  perichondrium  layer,  and  an 
inner  narrow  one,  along  which  a  similar  fusion  takes  place 
with  the  perichondrium  of  a  shelf-like  process  of  the  car- 
tilage, which  later  ossifies  to  form  the  lamina  spiralis. 
Consequently  throughout  the  cochlear  region  the  perilym- 
phatic space  is  divided  into  two  compartments  which  com- 
municate at  the  apex  of  the  cochlea,  while  below  one,  known 


470 


THE    INTERNAL    EAR. 


as  the  scala  vcstihuli,  communicates  with  the  space  surround- 
ing the  saccule  and  utricle,  and  the  other,  the  scala  tympani, 
abuts  upon  a  membrane  which  separates  it  from  the  cavity 
of  the  middle  ear  and  represents  a  portion  of  the  outer  wall 
of  the  petrous  bone  where  chondrification  and  ossification 
have  failed  to  occur.  This  membrane  closes  what  appears 
in  the  dried  skull  to  be  an  opening  in  the  inner  wall  of  the 
middle  ear,  known  as  the  fenestra  cochlecc  (rotunda)  ;  an- 
other similar  opening,  also  closed  by  membrane  in  the  fresh 


Fig.    254. — Diagrammatic   Transverse   Section   through    a   Coil   of 

THE  Cochlea,  showing  the  Relation  of  the  Scal^. 
c,  Organ  of   Corti ;    co,  ganglion  cochleare ;    Is,   lamina  spiralis ;   SM, 

cochlear    duct;    ST^    scala    tympani;    SP^,    scala    vestibuli. —  (From 

Gerlach.) 

skull,  occurs  in  the  bony  wall  opposite  the  utricular  portion 
of  the  otocyst  and  is  known  as  the  fenestra  vestibuli  (ovalis). 
The  Development  of  the  Middle  Ear. — The  middle  ear 
develops  from  the  upper  part  of  the  pharyngeal  groove 
which  represents  the  endodermal  portion  of  the  first 
branchial  cleft.     This  becomes  prolonged  dorsally  and  at 


THE    MIDDLE    EAR.  4/1 

its  dorsal  end  enlarges  to  form  the  tympanic  cavity,  while 
the  narrower  portion  intervening"  between  this  and  the 
pharyngeal  cavity  represents  the  tuba  anditiva  (Eustachian 
tube). 

To  correctly  understand  the  development  of  the  tympanic 
cavity  it  is  necessary  to  recall  the  structures  which  form  its 
boundaries.  Anteriorly  to  the  upper  end  of  the  first  branch- 
ial pouch  there  is  the  upper  end  of  the  first  arch,  and  behind 
it  the  corresponding  part  of  the  second  arch,  the  two  fusing 
together  dorsal  to  the  tympanic  cavity  and  forming  its  roof. 
Internally  the  cavity  is  bounded  by  the  outer  wall  of  the 
cartilaginous  investment  of  the  otocyst,  while  externally  it 
is  separated  from  the  upper  part  of  the  ectodermal  groove 
of  the  first  branchial  cleft  by  the  thin  membrane  which  forms 
the  floor  of  the  groove. 

It  has  been  seen  in  an  earlier  chapter  that  the  axial  meso- 
derm of  each  branchial  arch  gives  rise  to  skeletal  structures 
and  muscles.  The  axial  cartilage  of  the  ventral  portion  of 
the  first  arch  is  what  is  known  as  Meckel's  cartilage,  but  in 
that  portion  of  the  arch  which  forms  the  roof  and  anterior 
wall  of  the  tympanic  cavity,  the  cartilage  becomes  con- 
stricted to  form  two  masses  which  later  ossify  to  form  the 
malleus  and  incus  (Fig.  250,  m  and  i),  while  the  muscular 
tissue  of  this  dorsal  portion  of  the  arch  gives  rise  to  the 
tensor  tympani.  Similarly,  in  the  case  of  the  second  arch 
there  is  to  be  found,  dorsal  to  the  extremity  of  the  cartilag'e 
which  forms  the  styloid  process  of  the  adult,  a  narrow  plate 
of  cartilage  which  forms  an  investment  for  the  facial  nerve 
(Fig.  250,  VII),  and  dorsal  to  this  a  ring  of  cartilage  {st) 
which  surrounds  a  small  stapedial  artery  and  represents  the 
stapes. 

It  has  been  found  that  in  the  rabbit  the  mass  of  cells  from 
which  the  stapes  is  formed  is  at  its  first  appearance  cjuite 
independent  of  the  second  branchial  arch  (Fuchs),  and  it 


472  THE    MIDDLE    EAR. 

has  been  held  to  be  a  derivative  of  the  mesenchyme  from 
which  the  periotic  capsule  is  formed.  In  later  stages,  how- 
ever, it  becomes  connected  with  the  cartilage  of  the  second 
branchial  arch,  as  shown  in  Fig.  255,  and  it  is  a  question 
whether  this  connection,  which  is  transitory,  does  not  really 
indicate  the  phylogenetic  origin  of  the  ossicle  from  the  sec- 
ond arch  cartilage,  its  appearance  as  an  independent  struc- 


FiG.  255. — Semi-diagrammatic  View  of  the  Auditory  Ossicles  of  an 
Embryo  of  Six   Weeks. 

i.  Incus;  /,  jugular  vein;  ni,  malleus;  nic,  Meckel's  cartilage;  oc,  cap- 
sule of  otocyst ;  R,  cartilage  of  the  second  branchial  arch ;  st, 
stapes;   VII,  facial  nerve. —  (Siebenmann.) 

ture  being  a  secondary  ontogenetic  phenomenon.  However 
that  may  be,  the  stapedial  artery  disappears  in  later  stages 
and  the  stapedius  muscle,  derived  from  the  musculature  of 
the  second  branchial  arch  and  therefore  supplied  by  the 
facial  nerve,  becomes  attached  to  the  ossicle. 

The  three  ossicles  at  first  lie  embedded  in  the  mesenchyme 
forming  the  roof  of  the  primitive  tympanic  cavity,  as  does 
also  the  chorda  tympani,  a  branch  of  the  seventh  nerve,  as 


THE    MIDDLE   EAR. 


473 


it  passes  into  the  substance  of  the  first  arch  on  the  way  to 
its  destination.  The  mesenchyme  in  which  these  various 
structures  are  embedded  is  rather  voluminous  (Fig.  257), 
and  after  the  end  of  the  seventh  moiith  becomes  converted 

into  a  pecuHar  spongy  tissue,      ^ ■     ■  ..■^v 

which,  toward  the  end  of  fetal        '  \«». 

life,  gradually  degenerates,  the  '^.     ~  _^ 

tympanic   cavity   at    the    same  •' '■; 

time  expanding  and  wrapping  \,,  ..,•-'  . 

itself  around  the  ossicles  and 

the  muscles  attached  to  them     p"- ■■''';,:■-'--.,, ..,    '  '  :  ■• 

(Fig.    256).      The  bones   and  /        ,''^W 

their     muscles,     consequently,         m   i  "'■•—''  ; 

while  appearing  in  the  adult  to  ;  \ 

traverse   the   tympanic   cavity,  '■--. ..--' 

are  really  completely  enclosed 

within   a   layer   of   epithelium     ^  :  -  - -... 

continuous  with  that  lining  the  ■  /'  (^'\m 

wall  of  the  cavity,   while  the  ;  ""'"'''  i  • 

handle  of  the  malleus  and  the  \  ^ 

chorda  tympani  lie  between  the       ■'         '"■•■-•:------ '''  •     <? 

epithelium    of    the    outer    wall      F^^.     256.— Diagrams     Illus- 

r     ,1  •,  1     ,1  r^  TRATING    THE     MoDE     OF     Ex- 

ot   the  cavity  and   the  fibrous         tension   of  the  Tympanic 

mesoderm     which     forms     the         Cavity  Around  the  Audi- 
tory Ossicles. 
tympanic  membrane.  m,  Malleus ;  m,  spongy  mesen- 

The    extension    of    the    tvm-  chyme;    />,   inner   surface   of 

,  '  the  periotic  capsule;  T,  tym- 

panic cavity  does  not,  however,  panic    cavity.       The    broken 

cease  with  its  replacement  of         !i"fJ'Prfu"!-'  ^^'  epithelial 

ir  linmg  01  the  tympanic  cavity. 

the  degenerated  spongy  mesen- 
chyme, but  toward  the  end  of  fetal  life  it  begins  to  invade 
the  substance  of  the  temporal  bone  by  a  process  similar  to 
that  which  produces  the  ethmoidal  cells  and  the  other  osse- 
ous sinuses  in  connection  with  the  nasal  cavities  (see  p. 
186).  This  process  continues  for  some  years  after  birth 
41 


474  THE    MIDDLE    EAR. 

and  results  in  the  formation  in  the  mastoid  portion  of  the 
bone  of  the  so-called  mastoid  cells,  which  communicate  with 
the  tympanic  cavity  and  have  an  epithelial  lining  continuous 
with  that  of  the  cavity. 

The  lower  portion  of  the  diverticulum  from  the  first 
pharyngeal  groove  which  gives  rise  to  the  tympanic  cavity 
becomes  converted  into  the  Eustachian  tube.  During  devel- 
opment the  lumen  of  the  tube  disappears  for  a  time,  prob- 
ably owing  to  a  proliferation  of  its  lining  epithelium,  but 
it  is  re-established  before  birth. 

In  the  account  of  the  development  of  the  ear-bones  given 
above  it  is  held  that  the  malleus  and  incus  are  derivatives  of 
the  first  branchial  (mandibular)  arch  and  the  stapes  probably 
of  the  second.  This  view  represents  the  general  consensus  of 
recent  workers  on  the  difficult  question  of  the  origin  of  these 
bones,  but  it  should  be  mentioned  that  nearly  all  possible  modes 
of  origin  have  been  at  one  time  or  other  suggested.  The  mal- 
leus has  very  generally  been  accepted  as  coming  from  the  first 
arch,  and  the  same  is  true  of  the  incus,  although  some  earlier 
authors  have  assigned  it  to  the  second  arch.  But  with  regard 
to  the  stapes  the  opinions  have  been  very  varied.  It  has  been 
held  to  be  derived  from  the  first  arch,  from  the  second  arch, 
from  neither  one  nor  the  other,  but  from  the  cartilaginous  in- 
vestment of  the  otocyst,  or,  finally,  it  has  been  held  to  have  a 
compound  origin,  its  arch  being  a  product  of  the  second  arch 
while  its  basal  plate  was  a  part  of  the  otocyst  investment. 

The  Development  of  the  Tympanic  Membrane  and  of  the 
Outer  Ear. — Just  as  the  tympanic  cavity  is  formed  from  the 
endodermal  groove  of  the  first  branchial  cleft,  so  the  outer 
ear  owes  its  origin  to  the  ectodermal  groove  of  the  same 
cleft  and  to  the  neighboring  arches.  The  dorsal  and  most 
ventral  portions  of  the  groove  flatten  out  and  disappear, 
but  the  median  portion  deepens  to  form,  at  about  the  end 
of  the  second  month,  a  funnel-shaped  cavity  which  corre- 
sponds to  the  outer  portion  of  the  external  auditory  meatus. 
From  the  inner  end  of  this  a  solid  ingrowth  of  ectoderm 
takes  place,  and  this,  enlarging  at  its  inner  end  to  form  a 


THE    EXTERNAL    EAR, 


475 


disk-like  mass,  comes  into  relation  with  the  gelatinous  meso- 
derm which  surrounds  the  malleus  and  chorda  tympani. 
At  about  the  seventh  month  a  split  occurs  in  the  disk-like 
mass  (Fig.  257),  separating  it  into  an  outer  and  an  inner 
layer,  the  latter  of  which  becomes  the  outer  epithelium  of 
the  tympanic  membrane.     Later,  the  split  extends  outward 


Fig.  257. — Horizontal  Section  Passing  through  the  Dorsal  Wall 
OF  THE  External  Auditory  Meatus  in  an  Embryo  of  4.5  cm. 

c,  Cochlea ;  de,  endolymphatic  duct ;  i,  incus ;  Is,  transverse  sinus ;  m, 
malleus ;  me,  meatus  auditorius  externus ;  me',  cavity  of  the  meatus ; 
s,  sacculus ;  sc,  lateral  semicircular  canal ;  sc',  posterior  semi- 
circular canal;  st,  stapes;  t,  tympanic  cavity;  u,  utriculus;  7,  facial 
nerve. —  (Siebenmann.) 

in  the  substance  of  the  ectodermal  ingrowth  and  eventually 
unites  with  the  funnel-shaped  cavity  to  complete  the  exter- 
nal meatus. 


476 


THE   EXTERNAL    EAR. 


The  tympanic  membrane  is  formed  in  considerable  part 
from  the  substance  of  the  first  branchial  arch,  the  area  in 
which  it  occurs  not  being  primarily  part  of  the  wall  of  the 
tympanic  cavity,  but  being  brought  into  it  secondarily  by  the 
expansion  of  the  cavity.     The  membrane  itself  is  mesoder- 


% 


B 


«  E 


Fig.   258. — Stages   in   the  Development  of  the   Auricle. 

A,  Embryo  of  11  mm.;  B,  of  13.6  mm.;  C,  of  15  mm.;  D,  at  the  begin- 

ing  of  the  third  month;  E,  fetus  of  8.5  cm.;  F,  fetus  at  term. —  (His.) 


mal  in  origin  and  is  lined  on  its  outer  surface  by  an  ecto- 
dermal and  on  the  inner  by  an  endodermal  epithelium. 

The  auricle  (pinna)  owes  its  origin  to  the  portions  of  the 
first  and  second  arches  which  bound  the  entrance  of  the 
external  meatus.  Upon  the  posterior  edge  of  the  first  arch 
there  appear  about  the  end  of  the  fourth  week  two  trans- 


THE   EYE.  477 

verse  furrows  which  mark  off  three  tubercles  (Fig.  258,  A, 
1-3)  and  on  the  anterior  edge  of  the  second  arch  a  corre- 
sponding number  of  tubercles  (4-6)  is  formed,  Avhile,  in 
addition,  a  longitudinal  furrow,  running  down  the  middle 
of  the  arch,  marks  off  a  ridge  (c)  lying  posterior  to  the 
tubercles.  From  these  six  tubercles  and  the  ridge  are  devel- 
oped the  various  parts  of  the  auricle,  as  may  be  seen  from 
Fig.  258.  The  most  ventral  tubercle  of  the  first  arch  (i) 
gives  rise  to  the  tragus,  and  the  middle  one  (5)  of  the  sec- 
ond arch  furnishes  the  antitragiis.  The  middle  and  dorsal 
tubercles  of  the  first  arch  (2  and  3)  unite  with  the  ridge 
(c).to  produce  the  helix,  while  from  the  dorsal  tubercle  of 
the  second  arch  (4)  is  produced  the  anthelix  and  from  the 
ventral  one  (6)  the  lobule.  It  is  noteworthy  that  at  about 
the  third  month  of  development  the  upper  and  posterior 
portion  of  the  helix  is  bent  forward  so  as  to  conceal  the 
anthelix;  it  is  at  just  about  a  corresponding  stage  that  the 
pointed  form  of  the  ear  seen  in  the  lower  mammals  makes 
its  appearance,  and  it  is  evident  that,  were  it  not  for  the 
forward  bending,  the  human  ear  would  also  be  assuming 
at  this  stage  a  more  or  less  pointed  form.  Indeed,  there  is 
usually  to  be  found  upon  the  incurved  edge  of  the  helix, 
some  distance  below  the  upper  border  of  the  auricle,  a  more 
or  less  distinct  tubercle,  known  as  Damnn's  tubercle,  which 
seems  to  represent  the  point  of  the  typical  mammalian  ear, 
and  is,  accordingly,  the  morphological  apex  of  the  pinna. 

There  seems  to  be  little  room  for  doubt  that  the  otocyst 
belongs  primarily  to  the  system  of  lateral  line  sense-organs, 
but  a  discussion  of  this  interesting  question  would  necessitate 
a  consideration  of  derails  concerning  the  development  of  the 
lower  vertebrates  which  would  be  foreign  to  the  general  plan 
of  this  book.  It  may  be  recalled,  however,  that  the  analysis 
of  the  components  of  the  cranial  nerves  described  on  page  443 
refers  the  auditory  nerve  to  the  lateral  line  system. 

The  Development  of  the  Eye. — The  first  indications  of 


47^  THE    EYE, 

the  development  of  the  eye  are  to  be  found  in  a  pair  of 
hollow  outgrowths  from  the  side  of  the  first  primary  brain 
vesicle,  at  a  level  which  corresponds  to  the  junction  of  the 


Fig.  259. — Early  Stages  in  the  Development  of  the  Lens  in  a 
Rabbit    Embryo.     ' 

The  nucleated  layer  to  the  left  is  the  ectoderm  and  the  thicker  lens  epi- 
thelium, below  which  is  the  outer  wall  of  the  optic  evagination ; 
above  and  below  between  the  two  is  mesenchyme. —  (Rabl.) 

dorsal  and  ventral  zones.     Each  evagination  is  directed  at 
first  upward  and  backward,  and,  enlarging  at  its  extremity, 


THE   EYE. 


479 


it  soon  shows  a  differentiation  into  a  terminal  bulb  and  a 
stalk  connecting  the  bulb  with  the  brain  (Fig.  225).  At 
an  early  stage  the  bulb  comes  into  apposition  with  the  ecto- 
derm of  the  side  of  the  head,  and  this,  over  the  area  of  con- 
tact, becomes  thickened  and  then  depressed  to  form  the 
beg'inning  of  the  future  lens  (Fig\  259). 

As  the  result  of  the  depression  of  the  lens  ectoderm,  the 
outer  wall  of  the  optic  bulb  becomes  pushed  inward  toward 
the  inner  wall,  and  this  invagination  continuing  until  the 


Fig.    260. — Reconstruction    of   the    Brain   of   an    Embryo   of    Four 
Weeks,  showing  the  Chorioid  Fissure. — (His.) 


two  walls  come  into  contact,  the  bulb  is  transformed  into 
a  double-walled  cup,  the  optic  cup,  in  the  mouth  of  which 
lies  the  lens  (Fig.  261).  The  cup  is  not  perfect,  however, 
since  the  invagination  affects  not  only  the  optic  bulb,  but 
also  extends  medially  on  the  posterior  surface  of  the  stalk, 
forming  upon  this  a  longitudinal  groove  and  producing  a 
defect  of  the  ventral  wall  of  the  cup,  known  as  the  chorioidal 
fissure  (Fig.  260).  The  groove  and  fissure  become  occu- 
pied by  mesodermal  tissue,  and  in  this,  at  about  the  fifth 


480  THE    EYE, 

week,  a  blood-vessel  develops  which  traverses  the  cavity  of  the 
cup  to  reach  the  lens  and  is  known  as  the  arteria  hyaloidea. 

In  the  meantime  further  changes  have  been  taking  place 
in  the  lens.  The  ectodermal  depression  which  represents 
it  gradually  deepens  to  form  a  cup,  the  lips  of  which  ap- 
proximate and  finally  meet,  so  that  the  cup  is  converted 
into  a  vesicle  which  finally  separates  completely  from  the 
ectoderm  (Fig.  261),  much  in  the  same  way  as  the  otocyst 
does.  As  the  lens  vesicle  is  constricted  off,  the  surround- 
ing mesodermal  tissue  grows  in  to  form  a  layer  between 
it  and  the  overlying  ectoderm,  and  a  split  appearing  in  the 
layer  divides  it  into  an  outer  thicker  portion,  which  repre- 
sents the  cornea,  and  an  inner  thinner  portion,  which  covers 
the  outer  surface  of  the  lens  and  becomes  highly  vascular. 
The  cavity  between  these  two  portions  represents  the  anterior 
chamber  of  the  eye.  The  cavity  of  the  optic  cup  has  also 
become  filled  by  a  peculiar  tissue  which  represents  the  vit- 
reous humor,  while  the  mesodermal  tissue  surrounding  the 
cup  condenses  to  form  a  strong  investment  for  it,  which  is 
externally  continuous  with  the  cornea,  and  at  about  the  sixth 
week  shows  a  differentiation  into  an  inner  vascular  layer, 
the  chorioid  coat,  and  an  outer  denser  one,  which  becomes 
the  sclerotic  coat. 

The  various  processes  resulting  in  the  formation  of  the 
eye,  which  have  thus  been  rapidly  sketched,  may  now  be 
considered  in  greater  detail. 

The  Development  of  the  Lens. — When  the  lens  vesicle  is 
complete,  it  forms  a  more  or  less  spherical  sac  lying  beneath 
the  superficial  ectoderm  and  containing  in  its  cavity  a  few. 
cells,  either  scattered  or  in  groups  (Fig.  261).  These  cells, 
which  have  wandered  into  the  cavity  of  the  vesicle  from  its 
walls,  take  no  part  in  the  further  development  of  the  lens, 
but  early  undergo  complete  degeneration,  and  the  first  change 
which  is  concerned  with  the  actual  formation  of  the  lens  is 


THE    LENS. 


481 


an  increase  in  the  height  of  the  cells  forming  its  inner  wall 
and  a  thinning  out  of  its  outer  wall  (Fig.  262,  A).  These 
changes  continuing,  the  outer  half  of  the  vesicle  becomes 
converted  into  a  single  layer  of  somewhat  flat  cells  which 
persist  in  the  adult  condition  to  form  the  anterior  epitheliuin 
of  the  lens,  while  the  cells  of  the  posterior  wall  form  a 
marked  projection  into  the  cavity  of  the  vesicle  and  event- 


Fig.  261.— Horizontal  Section  through  the  Eye  of  an  Embryo  Pig 

OF   7    MM. 

B>%  Diencephalon ;   Ec,   ectoderm;   /,   lens;   P,  pigment,   and   R,   retinal 
layers   of   the   retina. 

ually  completely  obliterate  it,  coming  into  contact  with  the 
inner  surface  of  the  anterior  epithelium  (Fig.  262,  B). 
These  posterior  elongated  cells  form,  then,  the  principal 
42 


482  THE    LENS. 

mass  of  the  lens,  and  constitute  what  are  known  as  the  lens 
fibers.  At  first  those  situated  at  the  center  of  the  posterior 
wall  are  the  longest,  the  more  peripheral  ones  gradually 
diminishing  in  length  until  at  the  equator  of  the  lens  they 
become  continuous  with  and  pass  into  the  anterior  epithe- 
lium. As  the  lens  increases  in  size,  however,  the  most  cen- 
trally situated  cells  fail  to  elongate  as  rapidly  as  the  more 
peripheral  ones  and  are  pushed  in  toward  the  center  of  the 
lens,  the  more  peripheral  fibers  meeting  below  them  along  a 
line  passing  across  the  inner  surface  of  the  lens.  The  dis- 
parity of  growth  continuing,  a  similar  sutural  line  appears 
on  the  outer  surface  beneath  the  anterior  epithelium,  and  the 
fibers  become  arranged  in  concentric  layers  around  a  central 
core  composed  of  the  shorter  fibers.  In  the  human  eye  the 
line  of  suture  of  the  peripheral  fibers  becomes  bent  so  as  to 
consist  of  two  limbs  which  meet  at  an  angle,  and  from  the 
angle  a  new  sutural  line  develops  during  embryonic  life, 
so  that  the  suture  assumes  the  form  of  a  three-rayed  star. 
In  later  life  the  stars  become  more  complicated,  being  either 
six-rayed  or  more  usually  nine-rayed  in  the  adult  condition 
(Fig.  263). 

As  early  as  the  second  month  of  development  the  lens 
vesicle  becomes  completely  invested  by  the  mesodermal  tissue 
in  which  blood-vessels  are  developed  in  considerable  num- 
bers, whence  the  investment  is  termed  the  tunica  vasculosa 
lentis  (Fig.  271,  tv).  The  arteries  of  the  tunic  are  in  con- 
nection principally  with  the  hyaloid  artery  of  the  vitreous 
humor  (Fig.  269),  and  consist  of  numerous  fine  branches 
which  envelop  the  lens  and  terminate  in  loops  almost  at  the 
center  of  its  outer  surface.  This  tunic  undergoes  degenera- 
tion after  the  seventh  month  of  development,  by  which  time 
the  lens  has  completed  its  period  of  most  active  growth,  and, 
as  a  rule,  completely  disappears  before  birth.  Occasionally, 
however,  it  may  persist  to  a  greater  or  less  extent,  the  per- 


THE    LENS. 


483 


""^"u.. 


V, 


\ 


Fig.  262. — Sections  through  the  Lens  (A)  of  Human  Embryo  of 
Thirty  to  Thirty-one  Days  and  (B)  of  Pig  Embryo  of  36  mm. — 
(Rabl.) 


484 


THE    LENS. 


sistence  of  the  portion  covering  the  outer  surface  of  the  lens, 
known  as  the  memhrana  pupillaris,  causing  the  malformation 
known  as  congenital  atresia  of  the  pupil. 

In  addition  to  the  vascular  tunic,  the  lens  is  surrounded 


Fig.  263. — Posterior   (Inner)    Surface  of  the  Lens  from  an  Adult 

SHOWING    THE    SuTURAL    LiNES. —  (Rabl.) 


by  a  non-cellular  membrane  termed  the  capsule.  The  origin 
of  this  structure  is  still  in  doubt,  some  observers  maintaining 
that  it  is  a  product  of  the  investing  mesoderm,  while  others 
hold  it  to  be  a  product  of  the  lens  epithelium. 

It  is  interesting  from  the  standpoint  of  developmental  me- 
chanics to  note  that  W.  H.  Lewis  and  Spemann  have  shown 


THE    OPTIC    CUP.  485 

that  in  the  Amphibia  contact  of  the  optic  vesicle  with  the 
ectoderm  is  necessary  for  the  formation  of  the  lens,  and, 
furthermore,  if  the  vesicle  be  transplanted  to  other  regions  of 
the  body  of  a  larva,  a  lens  will  be  developed  from  the  ectoderm 
with  which  it  is  then  in  contact,  even  in  the  abdominal  region 

The  Development  of  the  Optic  Cup. — When  the  invagina- 
tion of  the  outer  wall  of  the  optic  bulb  is  completed,  the 
margins  of  the  resulting  cup  are  opposite  the  sides  of  the 
lens  vesicle  (Fig.  261),  but  with  the  enlargement  of  the  lens 
and  cup  the  margins  of  the  latter  gradually  come  to  lie  in 
front  of — that  is  to  say,  upon  the  outer  surface  of — the  lens, 
forming  the  boundary  of  the  opening  known  as  the  pupil. 
The  lens,  consequently,  is  brought  to  lie  within  the  mouth 
of  the  optic  cup,  and  that  portion  of  the  latter  which  covers 
the  lens  takes  part  in  the  formation  of  the  iris  and  the  adja- 
cent ciliary  body,  while  its  posterior  portion  gives  rise  to  the 
retina. 

The  chorioidal  fissure  normally  disappears  during  the 
sixth  or  seventh  week  of  development  by  a  fusion  of  its 
lips,  and  not  until  this  is  accomplished  does  the  term  cup 
truly  describe  the  form  assumed  by  the  optic  bulb  after  the 
invagination  of  its  outer  wall.  In  certain  cases  the  lips  of 
the  fissure  fail  to  unite  perfectly,  producing  the  defect  of  the 
eye  known  as  coloboma;  this  may  vary  in  its  extent,  some- 
times afl^ecting  both  the  iris  and  the  retina  and  forming 
what  is  termed  coloboma  iridis,  and  at  others  being  confined 
to  the  retinal  portion  of  the  cup,  in  which  case  it  is  termed 
coloboma  chorioideje. 

Up  to  a  certain  stage  the  difi^erentiation  of  the  two  layers 
which  form  the  optic  cup  proceeds  along  similar  lines,  in 
both  the  ciliary  and  retinal  regions.  The  layer  which  rep- 
resents the  original  internal  portion  of  the  bulb  does  not 
thicken  as  the  cup  increases  in  size,  and  becomes  also  the 
seat  of  a  deposition  of  dark  pigment,  whence  it  may  be 
termed  the  pigment  layer  of  the  cup;  while  the  other  layer — 


4^6  THE    IRIS    AND    CILIARY    BODY. 

that  formed  by  the  invagination  of  the  outer  portion  of  the 
bulb,  and  which  may  be  termed  the  retinal  layer — remains 
much  thicker  (Fig.  261)  and  in  its  proximal  portions  even 
increases  in  thickness.  Later,  however,  the  development  of 
the  ciliary  and  retinal  portions  of  the  retinal  layers  differs, 
and  it  will  be  convenient  to  consider  first  the.  history  of  the 
ciliary  portion. 

The  Development  of  the  Iris  and  Ciliary  Body. — The  first 
change  noticeable  in  the  ciliary  portion  of  the  retinal  layer 
is  its  thinning  out,  a  process  which  continues  until  the  layer 
consists,  like  the  pigment  layer,  of  but  a  single  layer  of  cells 
(Fig.  264),  the  transition  of  which  to  the  thicker  retinal 
portion  of  the  layer  is  somewhat  abrupt  and  corresponds  to 
what  is  termed  the  ora  serrata  in  adult  anatomy.  In  em- 
bryos of  10.2  cm.  the  retinal  layer  throughout  its  entire 
extent  is  readily  distinguishable  from  the  pigment  layer 
by  the  absence  in  it  of  all  pigmentation,  but  in  older  forms 
this  distinction  gradually  diminishes  in  the  iris  region,  the 
retinal  layer  there  acquiring  pigment  and  forming  the  uvea. 

When  the  anterior  chamber  of  the  eye  is  formed  by  the 
splitting  of  the  mesoderm  which  has  grown  in  between  the 
superficial  ectoderm  and  the  outer  surface  of  the  lens,  the 
peripheral  portions  of  its  posterior  (inner)  wall  are  in  rela- 
tion with  the  ciliary  portion  of  the  optic  cup  and  give  rise 
to  the  stroma  of  the  ciliary  body  and  of  the  iris  (Fig.  264), 
this  latter  being  continuous  with  the  tunica  vasculosa  lentis 
so  long  as  that  structure  persists  (Fig.  271).  In  embryos 
of  about  14.5  cm.  the  ciliary  portion  of  the  cup  becomes 
thrown  into  radiating  folds  (Fig.  264),  as  if  by  a  too  rapid 
growth,  and  into  the  folds  lamellae  of  mesoderm  project 
from  the  stroma.  These  folds  occur  not  only  throughout 
the  region  of  the  ciliary  body,  but  also  extend  into  the  iris 
region,  where,  however,  they  are  but  temporary  structures, 
disappearing  entirely  by  the  end  of  the  fifth  month.     The 


THE    IRIS    AND    CILIARY    BODY. 


487 


folds  in  the  region  of  the  corpus  cihare  persist  and  produce 
the  ciliary  processes  of  the  aduh  eye. 

Embedded  in  the  substance  of  the  iris  stroma  in  the  adult 
are  non-striped  muscle-fibers,  which  constitute  the  sphincter 
and  dilatator  iridis.  It  has  long  been  supposed  that  these 
fibers  were  differentiated  from  the  stroma  of  the  iris,  but 
recent  observations  have  shown  that  they  arise  from  the  cells 


Fig.    264. — Radial    Section    through    the    Iris    of    an    Embryo    of 

19   CM. 

AE,  Pigment  layer ;  CC,  ciliary  folds ;  IE,  retinal  layer ;  l.Str,  iris 
stroma ;  Pni,  pupillary  membrane ;  Rs,  marginal  sinus ;  Sph,  sphinc- 
ter iridis. —  {Szili.) 


of  the  pigment  layer  of  the  optic  cup,  the  sphincter  appear- 
ing near  the  pupillary  border  (Fig.  264,  Sph)  while  the  dila- 
tator is  more  peripheral. 

The  Development  of  the  Retina. — Throughout  the  retinal 
region  of  the  cup  the  pigment  layer,  undergoing  the  same 
changes  as  in  the  ciliary  region,  forms  the  pigment  layer 


488 


THE    RETINA. 


of  the  retina  (Fig.  265,  p).  The  retinal  layer  increases  in 
thickness  and  early  becomes  differentiated  into  two  strata 
(Fig.  261),  a  thicker  one  lying  next  the  pigment  layer  and 
containing'  numerous  nuclei,  and  a  thinner  one  containing'  no 
nuclei.  The  thinner  layer,  from  its  position  and  structure, 
suggests  an  homology  with  the  marginal  velum  of  the  cen- 
tral nervous  system,  and  probably  becomes  converted  into 


^°^O|6«6g(5°0(^ 


00  o^  ^  o 


o 


o 


Fig.    265. — Portion   of   a   Transverse   Section   of   the   Retina   of  a 

New-born  Rabbit. 
ch,  Chorioid  coat ;  g,  ganglion-cell  layer ;  r,  outer  layer  of  nuclei ;  p,  pig- 
ment  layer. —  (Falclii.) 

the  nerve-fiber  layer  of  the  adult  retina,  the  axis-cylinder 
processes  of  the  ganglion  cells  passing  into  it  on  their  way 
to  the  optic  nerve.  The  thicker  layer  similarly  suggests  a 
comparison  with  the  mantle  layer  of  the  cord  and  l)rain,  and 
in  embryos  of  38  mm.  it  becomes  differentiated  into  two 
secondary  layers  (Fig.  265),  that  nearest  the  pigment  layer 


THE    RETINA.  489 

(r)  consisting"  of  smaller  and  more  deeply  staining  nuclei, 
probably  representing  the  rod  and  cone  and  bipolar  cells  of 
the  adult  retina,  while  the  inner  layer,  that  nearest  the  mar- 
ginal velum,  has  larger  nuclei  and  is  presumably  composed 
of  the  ganglion  cells. 

Little  is  as  yet  known  concerning  the  further  differentia- 
tion of  the  nervous  elements  of  the  human  retina,  but  the 
history  of  some  of  them  has  been  traced  in  the  cat,  in  which, 
as  in  other  mammals,  the  histog'enetic  processes  take  place 
at  a  relatively  later  period  than  in  man.  Of  the  histogenesis 
of  the  inner  layer  the  information  is  rather  scant,  but  it 
may  be  stated  that  the  ganglion  cells  are  the  earliest  of  all 
the  elements  of  the  retina  to  become  recognizable.  The  rod 
and  cone  cells,  when  first  distinguishable,  are  unipolar  cells 
(Fig.  266,  a  and  c),  their  single  processes  extending  out- 
ward from  the  cell-bodies  to  the  external  limiting  membrane 
which  bounds  the  outer  surface  of  the  retinal  layer.  Even 
at  an  early  stage  the  cone  cells  (a)  are  disting-uishable  from 
the  rod  cells  (c)  by  their  more  decided  reaction  to  silver 
salts,  and  at  first  both  kinds  of  cells  are  scattered  throughout 
the  thickness  of  the  layer  from  which  they  arise.  Later,  a 
fine  process  grows  out  from  the  inner  end  of  each  cell,  which 
thus  assumes  a  bipolar  form  (Fig.  266,  b  and  d),  and,  later 
still,  the  cells  gradually  migrate  toward  the  external  limiting 
membrane,  beneath  which  they  form  a  definite  layer  in  the 
adult.  In  the  meantime  there  appears  opposite  the  outer 
end  of  each  cell  a  rounded  eminence  projecting  from  the 
outer  surface  of  the  external  limiting-  membrane  into  the 
pigment  layer.  The  eminences  over  the  cone  cells  are  larger 
than  those  over  the  rod  cells,  and  later,  as  both  increase  in 
length,  they  become  recognizable  by  their  shape  as  the  rods 
and  cones. 

The  bipolar  cells  are  not  easily  distinguishable  in  the 
early  stages  of  their  differentiation  from  the  other  cells 


490 


THE    RETINA. 


with  which  they  are  mingled,  but  it  is  beheved  that  they 
are  represented  by  cells  which  are  bipolar  Avhen  the  rod  and 
cone  cells  are  still  in  a  unipolar  condition  (Fig.  266,  e). 
If  this  identification  be  correct,  then  it  is  noteworthy  that 
at  first  their  outer  processes  extend  as  far  as  the  external 
limiting  membrane  and  must  later  shorten  or  fail  to  elon- 
erate  until  their  outer  ends  lie  in  what  is  termed  the  outer 


Fig.    266. — Diagram    showing    the    Development    of    the    Retinal 

Elements. 

a,  Cone  cell  in  the  unipolar,  and  b,  in  the  bipolar  stage;  c,  rod  cells  in 
the  unipolar,  and  d,  in  the  bipolar  stage ;  e,  bipolar  cells ;  f  and  i, 
amacrine  cells;  g,  horizontal  cells;  h,  ganglion  cells;  k,  Miiller's 
fiber;   I,  external  limiting  membrane. —  {Kallins,  after  Cajal.) 


granular  layer  of  the  retina,  where  they  stand  in  relation 
to  the  inner  ends  of  the  rod  and  cone  cell  processes.  Of 
the  development  of  the  amacrine  (/,  f)  and  horizontal  cells 
{g)  of  the  retina  little  is  known.  From  their  position  in 
new-born  kittens   it   seems   probable   that   the   former   are 


THE    OPTIC    NERVE.  49  ^ 

derived  from  cells  of  the  same  layer  as  the  ganglion  cells, 
while  the  horizontal  cells  may  belong  to  the  outer  layer. 

In  addition  to  the  various  nerve-elements  mentioned 
above,  the  retina  also  contains  neuroglial  elements  known 
as  Muller's  fibers  (Fig.  266,  k),  which  traverse  the  entire 
thickness  of  the  retina.  The  development  of  these  cells 
has  not  yet  been  thoroughly  traced,  but  they  resemble  closely 
the  ependymal  cells  observable  in  early  stages  of  the  spinal 
cord. 

The  Development  of  the  Optic  Nerve. — The  observations 
on  the  development  of  the  retina  have  shown  very  clearly 
that  the  great  majority  of  the  fibers  of  the  optic  nerve  are 
axis-cylinders  of  the  ganglion  cells  of  the  retina  and  grow 
from  these  cells  along  the  optic  stalk  toward  the  brain. 
Their  embryonic  history  has  been  traced  most  thoroughly 
in  rat  embryos  (Robinson),  and  what  follows  is  based  upon 
what  has  been  observed  in  that  animal. 

The  optic  stalk,  being  an  outgrowth  from  the  brain,  is  at 
first  a  hollow  structure,  its  cavity  communicating  with  that 
of  the  third  ventricle  at  one  end  and  with  that  of  the  optic 
bulb  at  the  other.  When  the  chorioid  fissure  is  developed, 
it  extends,  as  has  already  been  described,  for  some  distance 
along  the  posterior  surface  of  the  stalk  and  has  lying  in  it 
a  portion  of  the  hyaloid  artery.  Later,  when  the  lips  of 
the  fissure  fuse,  the  artery  becomes  enclosed  within  the  stalk 
to  form  the  arteria  centralis  retincc  of  the  adult  (Fig.  269). 
By  the  formation  of  the  fissure  the  original  cavity  of  the 
distal  portion  of  the  stalk  becomes  obliterated,  and  at  the 
same  time  the  ventral  and  posterior  walls  of  the  stalk  are 
brought  into  continuity  with  the  retinal  layer  of  the  optic 
cup,  and  so  opportunit}^  is  given  for  the  passage  of  the  axis- 
cylinders  of  the  ganglion  cells  along  those  walls  (Fig.  267). 
At  an  early  stage  a  section  of  the  proximal  portion  of  the 
optic  stalk    (Fig.   268,  A)    shows  the  central  cavity  sur- 


492 


THE    OPTIC    NERVE. 


roiindetl  by  a  number  of  nuclei  representing-  the  mantle  layer, 
and  surrounding-  these  a  non-nucleated  layer,  resembling  the 
marginal  velum  and  continuous  distally  with  the  similar 
layer  of  the  retina.  When  the  ganglion  cells  of  the  latter 
begin  to  send  out  their  axis-cylinder  processes,  these  pass 

into  the  retinal  marginal  velum 
and  converge  in  this  layer  toward 
the  bottom  of  the  ciliary  fis- 
sure, so  reaching  the  ventral  wall 
of  the  optic  stalk,  in  the  velum 
of  which  they  may  be  distin- 
guished in  rat  embryos  of  4 
mm.,  and  still  more  clearly  in 
those  of  9  mm.  (Fig.  268,  A). 
Fig.    267.  — Diagrammatic    Fater,  as  the  fibers  become  more 

Longitudinal       Section  numerous,  they  gradually  invade 
OF   THE   Optic   Cup   and  r' 

Stalk  passing  through  the  lateral  and  finally  the  dorsal 

THE  Chorioid  Fissure.  ^^^^    ^f    ^j^^    g^^lj^^    ^^^^^    ^^    ^j^^ 

^/z,  Hyaloid  artery ;  L,  lens ;  .  ,  ^i  n  r 

On,   fibers   of   the   optic   same    tnne    the    mantle    cells    of 

the  stalk  become  more  scattered 

and  assume  the  form  of  connec- 


nerve;  Os,  optic  stalk; 
PI,  pigment  layer,  and  R, 
retinal  layer  of  the  retina 


tive-tissue  (neuroglia)  cells,  while 

the   original   cavity   of   the   stalk   is   gradually   obliterated 

(Fig.   268,  B).     Finally,  the  stalk  becomes  a  soHd  mass 

of  nerve-fibers,  among  which  the  altered  mantle  cells  arc 

scattered. 

From  what  has  been  stated  above  it  will  be  seen  that  the 
sensory  cells  of  the  eye  belong  to  a  somewhat  different  cate- 
gory from  those  of  the  other  sense-organs.  Embryologically 
they  are  a  specialized  portion  of  the  mantle  layer  of  the  medul- 
lary canal,  whereas  in  the  other  organs  they  are  peripheral 
structures  either  representing  or  being  associated  with  repre- 
sentatives of  posterior  root  ganglion  cells.  Viewed  from  this 
standpoint,  and  taking  into  consideration  the  fact  that  the 
sensory  portion  of  the  retina  is  formed  from  the  invaginated 
part  of  the  optic  bulb,"  some  light  is  thrown  upon  the  inverted 


THE    OPTIC    NERVE.  493 

arrangement  of  the  retinal  elements,  the  rods  and  cones  behig 
directed  away  from  the  source  of  light.  The  normal  relations 
of  the  mantle  layer  and  marginal  velum  are  retained  in  the 
retina,  and  the  latter  serving  as  a  conducting  layer  for  the  axis- 
cylinders  of  the  mantle  layer  (ganglion)  cells,  the  layer  of 
nerve-fibers  becomes  interposed  between  the  source  of  light 
and  the  sensory  cells.  Furthermore,  it  may  be  pointed  out 
that  if  the  differentiation  of  the  retina  be  imagined  to  take 
place  before  the  closure  of  the  medullary  canal, — a  condition 
which   is  indicated  in  some  of  the   lower  vertebrates, — there 


e^ 


-"-Qb 


"<it._:.-^:^i:^ 


B 

Fig.  268. — Transverse  Sections  through  the  Proximal  Part  of 
THE  Optic  Stalk  of  Rat  Embryos  of  {A)  9  mm.  and  (5)  11  mm. 
—  {Robinson.) 

would  be  then  no  inversion  of  the  elements,  this  peculiarity 
being  due  to  the  conversion  of  the  medullary  plate  into  a  tube, 
and  more  especially  to  the  fact  that  the  retina  develops  from 
the  outer  wall  of  the  optic  cup.  In  certain  reptiles  in  which 
an  eye  is  developed  in  connection  with  the  epiphysial  out- 
growths of  the  diencephalon,  the  retinal  portion  of  this  pineal 
eye  is  formed  from  the  inner  layer  of  the  bulb,  and  in  this  case 
there  is  no  inversion  of  the  elements. 

A  justification  of  the  exclusion  of  the  optic  nerve  from  the 
category  which  includes  the  other  cranial  nerves  has  now  been 
presented.  For  if  the  retina  be  regarded  as  a  portion  of  the 
central  nervous  system,  it  is  clear  that  the  nerve  is  not  a  nerve 
at  all  in  the  strict  sense  of  that  word,  but  is  a  tract,  confined 
throughout  its  entire  extent  within  the  central  nervous  system 
and  comparable  to  such  groups  of  fibers  as  the  direct  cerebellar 
or  fillet  tracts  of  that  system. 


494  THE    VITREOUS    HUMOR. 

The  Development  of  the  Vitreous  Humor. — it  has  already 
been  pointed  out  (p.  480)  that  a  blood-vessel,  the  hyaloid 
artery,  accompanied  by  some  mesodermal  tissue  makes  its 
way  into  the  cavity  of  the  optic  cup  through  the  chorioid 
fissure.  On  the  closure  of  the  fissure  the  artery  becomes 
enclosed  within  the  optic  stalk  and  appears  to  penetrate  the 
retina,  upon  the  surface  of  which  its  branches  ramify.  In 
the  embryo  the  artery  does  not,  however,  terminate  in  these 
branches  as  it  does  in  the  adult,  but  is  continued  on  throueh 


Fig.  269. — Reconstruction  of  a   Portion  of  the  Eye  of  an  Embryo 

OF    13.8    MM. 

ah,  Hyaloid  artery;   ch,  chorioid  coat;   I,  lens;   r,   retina. —  (His.) 

the  cavity  of  the  optic  cup  (Fig.  269)  to  reach  the  lens, 
around  which  it  sends  branches  to  form  the  tunica  vasculosa 
lentis. 

According  to  some  authors,  the  formation  of  the  vitreous 
humor  is  closely  associated  with  the  development  of  this 
artery,  the  humor  being  merely  a  transudate  from  it,  while 
others  have  maintained  that  it  is  a  derivative  of  the  meso- 
derm which  accompanies  the  vessel,  and  is  therefore  to  be 
regarded  as  a  peculiar  gelatinous  form  of  connective  tissue. 
More  recently,  however,  renewed  observations  by  several 
authors  have  resulted  in  the  deposition  of  the  mesoderm 
from  the  chief  role  in  the  formation  of  the  vitreous  and 
the  substitution  in  it  of  tlie  retina.     At  an  early  stage  of 


THE    VITREOUS    HUMOR. 


495 


development  delicate  protoplasmic  processes  may  be  seen 
projecting  from  the  surface  of  the  retinal  layer  into  the 
cavity  of  the  optic  cup,  these  processes  probably  arising 
from  those  cells  which  will  later  form  the  Miiller's  (neurog- 
lia) fibres  of  the  retina.  As  development  proceeds  they 
increase  in  length,  forming  a  dense  and  very  fine  fibrillar 
reticulum  traversing  the  space  between  the  lens  and  the 


Fig.  2/0. — Transverse  Section  through  the  Ciliary  Region  of  a 
Chick  Embryo  of  Sixteen  Days. 

ac,  Anterior  chamber  of  the  eye;  cj,  conjunctiva;  co,  cornea;  i,  iris; 
I,  lens;  mc,  ciliary  muscle;  rl,  retinal  layer  of  optic  cup;  sf,  spaces 
of  Fontana ;  si,  suspensory  ligament  of  the  lens ;  v,  vitreous  humor. 
— (Angelucci.) 

retina  and  constituting  the  primary  vitreous  humor.  The 
formation  of  the  fibers  is  especially  active  in  the  ciliary  por- 
tion of  the  retina  and  it  is  probable  that  it  is  from  some  of 
the  fibers  developing  in  this  region  that  the  suspensory  liga- 
ment of  the  lens  {aomila  Zinnii)  (Fig.  270,  si)  is  formed, 
spaces  which  occur  between  the  fibers  of  the  ligament  enlarg- 


49^  THE    VITREOUS    HUMOR. 

ing  to  produce  a  cavity  traversed  by  scattered  fibers  and 
known  as  the  canal  of  Petit. 

A  participation  of  similar  protoplasmic  prolongations 
from  the  cells  of  the  lens  in  the  formation  of  the  vitreous 
humor  has  been  maintained  (von  Lenhossek)  and  as  strenu- 
ously denied.  But  it  is  generally  admitted  that  at  the  time 
when  the  hyaloid  artery  penetrates  the  vitreous  to  form  the 
tunica  vasculosa  lentis  it  carries  with  it  certain  mesodermal 
elements,  whose  fate  is  at  present  uncertain.  It  has  been 
held  that  they  take  part  in  the  formation  of  the  definitive 
vitreous,  which,  according  to  this  view,  is  of  mixed  origin, 
being  partly  ectodermal  and  partly  mesodermal  (Van  Pee), 
and,  on  the  contrary,  it  has  been  maintained  that  they  even- 
tually undergo  complete  degeneration,  the  vitreous  being  of 
purely  ectodermal  origin  (von  Kolliker). 

The  degeneration  of  the  mesodermal  elements  which  the 
latter  view  supposes  is  associated  with  the  degeneration  of 
the  hyaloid  artery.  This  begins  in  human  embryos  in  the 
third  month  and  is  completed  during  the  ninth  month,  the 
only  trace  after  birth  of  the  existence  of  the  vessel  being  a 
more  fluid  consistency  of  the  axis  of  the  vitreous  humor, 
this  more  fluid  portion  representing  the  space  originally 
occupied  by  the  artery  and  forming  what  is  termed  the  hya- 
loid canal  (canal  of  Cloqiiet). 

The  Development  of  the  Outer  Coat  of  the  Eye,  of  the 
Cornea,  and  of  the  Anterior  Chamber. — Soon  after  the 
formation  of  the  optic  bulb  a  condensation  of  the  meso- 
derm cells  around  it  occurs,  forming  a  capsule.  Over  the 
medial  portions  of  the  optic  cup  the  further  differentiation 
of  this  capsule  is  comparatively  simple,  resulting  in  the  for- 
mation of  two  layers,  an  inner  vascular  and  an  outer  denser 
and  fibrous,  the  former  Ijecoming  the  chorioid  coat  of  the 
adult  eye  and  the  latter  the  sclera. 

More  laterally,  however,  the  processes  are  more  compli- 


THE    CORNEA. 


497 


catecl.  After  the  lens  has  separated  from  the  surface  ecto- 
derm a  thin  layer  of  mesoderm  grows  in  between  the  two 
structures  and  later  gives  place  to  a  layer  of  homogeneous 
substance  in  which  a  few  cells,  more  numerous  laterally  than 
at  the  center,  are  embedded.  Still  later  cells  from  the  adja- 
cent mesenchyme  grow  into  the  layer,  which  increases  con- 
siderably in  thickness,  and  blood-vessels  also  gTow  into  that 
portion  of  it  which  is  in  contact  with  the  outer  surface  of  the 


ac 


Fig.   271. — Transverse   Section   through    the    Ciliary   Region   of   a 

Pig   Embryo  of  23    mm. 
ac,  Anterior  chamber  of' the  eye;  co,  cornea;  ec,  ectoderm;  /,  lens;  mc, 

ciliary  muscle;  p,  pigment  layer  of  the  optic  cup;  r,  retinal  layer; 

tv,  tunica  vasculosa  lentis. —  (Angchicci.) 

lens.  At  this  stage  the  interval  between  the  surface  ecto- 
derm and  the  lens  is  occupied  by  a  solid  mass  of  mesodermal 
tissue  (Fig.  271,  co  and  tv) ,  but  as  development  proceeds, 
small  spaces  filled  with  fluid  begin  to  appear  toward  the 
inner  portion  of  the  mass  (ac) ,  and  these,  increasing  in 
number  and  size,  eventually  fuse  together  to  form  a  single 
43 


498  THE   ANTERIOR   CHAMBER   OF   THE   EYE. 

cavity  which  divides  the  mass  into  an  inner  and  an  outer 
portion.  The  cavity  is  the  anterior  chamber  of  the  eye,  and 
it  has  served  to  separate  the  cornea  (co)  from  the  tunica 
vasculosa  lentis  (tv),  and,  extending  laterally  in  all  direc- 
tions, it  also  separates  from  the  cornea  the  mesenchyme 
which  rests  upon  the  marginal  portion  of  the  optic  cup  and 
constitutes  the  stroma  of  the  iris.  Cells  arrange  themselves 
on  the  corneal  surface  of  the  cavity  to  form  a  continuous 
endothelial  layer,  and  the  mesenchyme  which  forms  the 
peripheral  boundary  of  the  cavity  assumes  a  fibrous  charac- 
ter and  forms  the  ligamentum  pectinaHim  iridis,  among  the 
fibers  of  which  cavities,  known  as  the  spaces  of  Fontana 
(Fig.  270,  sf),  appear.  Beyond  the  margins  of  the  cavity 
the  corneal  tissue  is  directly  continuous  with  the  sclerotic, 
beneath  the  margin  of  which  is  a  distinctly  thickened  por- 
tion of  mesenchyme  resting  upon  the  ciliary  processes  and 
forming  the  stroma  of  the  ciliary  body,  as  well  as  giving 
rise  to  the  muscle  tissue  which  constitutes  the  ciliary  muscle 
(Figs.  270  and  271,  mc). 

The  ectoderm  which  covers  the  outer  surface  of  the  eye 
does  not  proceed  beyond  the  stage  when  it  consists  of  sev- 
eral layers  of  cells,  and  never  develops  a  stratum  corneum. 
In  the  corneal  region  it  rests  directly  upon  the  corneal  tis- 
sue, which  is  thickened  slightly  upon  its  outer  surface  to 
form  the  anterior  elastic  lamina;  more  peripherally,  how- 
ever, a  cjuantity  of  loose  mesodermal  tissue  lies  between  it 
and  the  outer  surface  of  the  sclerotic,  and,  together  with 
the  ectoderm,  forms  the  conjunctiva  (Fig.  265,  cj). 

The  Development  of  the  Accessory  Apparatus  of  the  Eye. 
— The  eyelids  make  their  appearance  at  an  early  stage  as 
two  folds  of  skin,  one  a  short  distance  above  and  the  other 
below  the  cornea.  The  center  of  the  folds  is  at  first  occu- 
pied by  indifferent  mesodermal  tissue,  which  later  becomes 
modified  to  form  the  connective  tissue  of  the  lids  and  the 


THE    EYELIDS. 


499 


tarsal  cartilage,  the  muscle  tissue  probably  secondarily 
growing  into  the  lids  as  a  result  of  the  spreading  of  the 
platysma  over  the  face,  the  orbicularis  oculi  apparently  being 
a  derivative  of  that  sheet  of  muscle  tissue. 

At  about  the  beginning  of  the  third  month  the  lids  have 
become  sufficiently  larg^e  to  meet  one  another,  whereupon 
the  thickened  epithelium  which  has  formed  upon  their  edges 
unites  and  the  lids  fuse  together,  in  which  condition  they 


Fig.  272. — Section  through  the  Margins  of  the  Fused  Eyelids  in 

AN  Embryo  of  Six   Months. 

h,   Eyelash ;    //,   lower   lid ;   m,   tarsal   gland ;    mu,  muscle  bundle ;   ul, 

upper   lid. —  {Schweigger-Seidl. ) 

remain  until  shortly  before  birth.  During  the  stage  of 
fusion  the  eyelashes  (Fig.  272,  h)  develop  at  the  edges  of 
the  lids,  having  the  same  developmental  history  as  ordinary 
hairs,  and  from  the  fused  epithelium  of  each  lid  there  grow- 
upward  or  downward,  as  the  case  may  be,  into  the  meso- 


500  THE    LACHRYMAL    GLAND. 

dermic  tissue,  solid  rods  of  ectoderm,  certain  of  which  early 
give  off  numerous  short  lateral  processes  and  become  recog- 
nizable as  the  tarsal  {Meibomian')  glands  (w),  while  others 
retain  the  simple  cylindrical  form  and  represent  the  glands 
of  Moll.  When  the  eyelids  separate,  these  solid  ingrowths 
become  hollow  by  a  breaking  down  of  their  central  cells, 
just  as  in  the  sebaceous  and  sudoriparous  glands  of  the  skin, 
the  tarsal  glands  being  really  modifications  of  the  former 
glands,  while  the  glands  of  Moll  are  probably  to  be  regarded 
as  specialized  sudoriparous  glands. 

A  third  fold  of  skin,  in  addition  to  the  two  which  pro- 
duce the  eyelids,  is  also  developed  in  connection  with  the 
eye,  forming  the  plica  semilunaris.  This  is  a  rudimentary 
third  eyelid,  representing  the  nictitating  membrane  which 
is  fairly  well  developed  in  many  of  the  lower  mammals  and 
especially  well  in  birds. 

The  lachrymal  gland  is  developed  at  about  the  third 
month  as  a  number  of  branching  outgrowths  of  the  ecto- 
derm into  the  adjacent  mesoderm  along  the  outer  part  of 
the  line  where  the  epithelium  of  the  conjunctiva  becomes 
continuous  with  that  covering  the  inner  surface  of  the  upper 
eyelid.  As  in  the  other  epidermal  glands,  the  outgrowths 
and  their  branches  are  at  first  solid,  later  becoming  hollow 
by  the  degeneration  of  their  axial  cells. 

The  naso-lachrymal  duct  is  developed  in  connection  with 
the  groove  which,  at  an  early  stage  in  the  development 
(Fig.  52),  extends  from  the  inner  corner  of  the  eye  to  the 
olfactory  pit  and  is  bounded  posteriorly  by  the  maxillary 
process  of  the  first  visceral  arch.  The  epithelium  lying  in 
the  floor  of  this  groove  thickens  toward  the  beginning  of 
the  sixth  week  to  form  a  solid  cord,  which  sinks  into  the 
subjacent  mesoderm,  though  retaining  connection  with  the 
ectoderm  of  the  olfactory  pit  at  its  lower  end.  From  its 
upper  end  two  outgrowths  arise  which  become  connected 


LITERATURE.  501 

with  the  ectoderm  of  the  edges  of  the  upper  and  lower  hds, 
respectively,  and  represent  the  lachrymal  ducts,  and,  finally, 
the  solid  cord  and  its  outgrowths  acquire  a  lumen. 

The  inferior  duct  connects  with  the  border  of  the  eyelid 
some  distance  lateral  to  the  inner  angle  of  the  eye,  and 
between  its  opening  and  the  angle  a  number  of  tarsal  g'lands 
develop.  The  superior  duct,  on  the  other  hand,  opens  at 
first  close  to  the  inner  angle  and  later  moves  laterally  until 
its  opening  is  opposite  that  of  the  inferior  duct.  During 
this  change  the  portion  of  the  lower  lid  between  the  opening 
of  the  inferior  duct  and  the  angle  is  drawn  somewhat  up- 
wards, and,  with  its  glands,  forms  a  small  reddish  nodule, 
resting  upon  the  plica  semilunaris  and  known  as  the  caniii- 
ciila  lacriuialis. 

LITERATURE. 

G.  Alexander  :  "  Ueber  Entwicklung  und  Bau  des  Pars  inferior  Laby- 
rinthi  der  hoheren  Saugethiere,"  Denkschr.  kais.  zvissench.  Acad. 
Wien,  Mafh.-Naturw.    Classe,   lxx,    1901. 

A.  Angelucci  :  "  Ueber  Entwickelung  und  Bau  des  vorderen  Uveal- 
tractus  der  Vertebraten,"  Archiv  fur  mikrosk.  Anat,  xix,  1881. 

F.  Ask  :  "  Ueber  die  Entwickelung  der  Caruncula  lacrimalis  beim  Men- 
schen,  nebst  Bemerkungen  iiber  die  Entwickelung  der  Tranen- 
rohrchen  und  der  Meibom'schen  Driisen,"  Anatoin.  Anzeiger,  xxx, 
1907. 

B.  Baginsky  :  "  Zur  Entwickelung  der  Gehorschnecke,"  Archiv  filr 
mikrosk.  Anat.,  xxviii,  1886. 

I.  Broman  :  "  Die  Entwickelungsgeschichte  der  Gehorknochelchen  beim 
Menschen,"  Anat.  Hefte,  xi,  1898. 

S.  Ramon  y  Cajal  :  "  Nouvelles  contributions  a  I'etude  histologique  de 
la  retine,"/oMrM.  de  I'Anat.  et  de  la  Physiol.,  xxxii,  1896. 

J.  Disse:  "Die  erste  Entwickelung  der  Riechnerven,"  Anat.  Hefte, 
IX,  1897. 

B.  Fleischer:  "Die  Entwickelung  der  Tranenrohrchen  bei  den  Sauge- 
tiere,"  Archiv  filr  Ophthalmol.,  lxii,  1906. 

H.  FucHS :  "  Bemerkungen  iiber  die  Herkunft  und  Entwickelung  der 
Gehorknochelchen  bei  Kaninchen-Embryonen  (nebst  Bemerkungen 
iiber  die  Entwickelung  des  Knorpelskeletes  der  beiden  ersten 
Visceralbogen),"  Archiv  fiir  Anat.  und  Phys.,  Anat.  Abth.,  Supple- 
ment, 1905. 


502  LITERATURE. 

J.   Graberg  :    "  Beitrage  zur   Genese   des   Geschmacksorgans   der   Men- 

schen,"  Morphol.  Arheiten,  vii,  1898. 
J.  A.  Hammar:  "Zur  allgemeinen  Morphologic  der  Schlundspalten  des 

Menschen.      Zur  Entwickelungsgeschichte  des  Mittelohrraumes,  des 

ausseren  Gehorganges  und  des  Paukenfelles  beim  Menschen,"  Anat. 

Anseiger,  xx,  1901. 
Heerfordt:  "  Studien  iiber  den  Muse,  dilatator  pupillas  sammt  Angabe 

von  gemeinschaftlicher  Kennzeichen  einiger  Falle  epithelialer  Mus- 

culatur,"  Anat.  Hefte,  xrv. 
J.    Hegetschweiler  :    "  Die    embryologische    Entwickelung    des    Steig- 

bugels,"  Archiv  fur  Anat.  und  Physiol.,  Anat.  Abth.,  i8g8. 

F.  Hochstetter:   "Ueber  die  Bildung  der  primitiven   Choanen  beim 

Menschen,"  Verhandl.  Anat.  Gesellsch.,  vi,  1892. 
W.   His,  Jr.  :   "  Die   Entwickelungsgeschichte   des  Acustico-Facialisge- 

bietes  beim  Menschen,"  Archiv  fi'ir  Anat.  und  Physiol.,  Anat.  Abth., 

Supplement,  1897. 
A.  VON  KoLLiKER :  "  Die  Entwicklung  und  Bedeutung  des  Glaskorpers," 

Zeitschr.  fiir  zmsscnsch.  Zoolog.,  lxxvi,  1904. 
V.  VON  MiHALKOVicz :  "  Nasenhohle  und  Jacobsonsches  Organ.     Fine 

morphologische  Studie,"  Anat.  Hefte,  xi,  1898. 
P.  VAN   Pee:   "  Recherches   sur  I'origine  du   corps  vitre,"  Archives  dc 

Biol.,  XIX,  1902. 
C.  Rabl  :   "  Ueber  den   Bau  und   Entwickelung  der  Linse,"  Zeitschrift 

fur  wissensch.  Zoologie,  lxii  and  lxv,  1898;  lxvii,  1899. 
A.  Robinson  :  "  On  the  Formation  and  Structure  of  the  Optic  Nerve 

and  Its  Relation  to  the  Optic  Stalk,"  Journal  of  Anat.  and  Physiol., 

XXX,  1896. 

G.  L.  Streeter  :  "  On  the  Development  of  the  Membranous  Labyrinth 

and  the  Acoustic  and  Facial  Nerves  in  the  Human  Embryo,"  Amer. 

Joiirn.  of  Anat.,  vi,  1907. 
A.   SziLi :    "Zur  Anatomic   und    Entwickelungsgeschichte   der   hinteren 

Irisschichten,     mit     besonderer     Beritcksichtigung     des     Musculus 

sphincter   iridis   des   Menschen,"   Anat.   Anzciger,   xx,    1901. 
F.  Tuckerman  :  "  On  the  Development  of  the  Taste  Organs  in  Man," 

Journal  of  Anat.  and  Physiol.,  xxiv,   1889. 


CHAPTER  XVII. 

POST-NATAL    DEVELOPMENT. 

In  the  preceding-  pages  attention  has  been  directed  princi- 
pally to  the  changes  which  take  place  in  the  various  organs 
during  the  period  before  birth,  for,  with  a  few  exceptions, 
notably  that  of  the  liver,  the  general  form  and  histological 
peculiarities  of  the  various  organs  are  acquired  before  that 
epoch.  Development  does  not,  however,  cease  vrith  birth, 
and  a  few  statements  regarding  the  changes  which  take 
place  in  the  interval  between  birth  and  maturity  will  not  be 
out  of  place  in  a  work  of  this  kind. 

The  conditions  which  obtain  during  embryonic  life  are 
so  different  from  those  to  which  the  body  must  later  adapt 
itself,  that  arrangements,  such  as  those  connected  with  the 
placental  circulation,  which  are  of  fundamental  importance 
during  the  life  in  iitcro,  become  of  little  or  no  use,  while 
the  relative  importance  of  others  is  greatly  diminished,  and 
these  changes  react  more  or  less  profoundly  on  all  parts  of 
the  body.  Hence,  although  the  post-natal  development  con- 
sists chiefly  in  the  growth  of  the  structures  formed  during 
earlier  stages,  yet  the  growth  is  not  equally  rapid  in  all 
parts,  and  indeed  in  some  organs  there  may  even  be  a  rela- 
tive decrease  in  size.  That  this  is  true  can  be  seen  from  the 
annexed  figure  (Fig.  273),  which  represents  the  body  of  a 
child  and  that  of  an  adult  man  drawn  as  of  the  same  height. 
The  greater  relative  size  of  the  head  and  upper  part  of  the 
body  in  the  child  is  very  marked,  and  the  central  point  of 
the  height  of  the  child  is  situated  at  about  the  level  of  the 
umbilicus,  while  in  the  man  it  is  at  the  symphysis  pubis. 

503 


504 


POST-NATAL    DEVELOPMENT. 


This  excessive  development  of  the  upper  portions  of  the 
body  of  the  child  is  largely  due  to  the  nature  of  the  blood- 
supply  during  fetal  life,  when,  as  may  be  seen  by  reference 
to  Fig.  157,  the  blood  passing  to  the  head,  neck,  arms  and 
upper  portions  of  the  thorax  leaves  the  aorta  before  the  duc- 
tus arteriosus  opens  into  it,  and  is  therefore  practically  un- 


FiG.  2']T). — Child  and  Man  Drawn  as  of  the  Same  Height. —  {hanger, 
from  the  "Growth  of  the  Brain,"  Contemporary  Science  Series, 
by  permission  of  Charles  Scribner's  Sons.) 

mixed  with  venous  blood,  while  throughout  the  rest  of  the 
body  the  supply  is  largely  diluted  with  blood  from  the  right 
side  of  the  heart. 

That  there  is  a  distinct  change  in  the  geometric  form  of 
the  body  during  growth  is  also  well  shown  by  the  following 
consideration  (Thoma).  Taking  the  average  height  of  a 
new-born  male  as  500  mm.,  and  that  of  a  man  of  thirty 
years  of  age  as  1686  mm.,  the  height  of  the  body  will  have 


POST-NATAL    DEVELOPMENT. 


505 


increased   from  birth  to  adolescence 


1686 
500 


3.37  times. 


The  child  will  weigh  3.1  kilos  and  the  man  66.1  kilos,  and 
if  the  specific  gravity  of  the  body  with  the  included  gases 
be  taken  in  the  one  case  as  0.90  and  in  the  other  as  0.93, 
then  the  volume  of  the  child's  body  will  be  3.44  liters  and 
that  of  the  man's  71.08  liters,  and  the  increase  in  volume 
71.08 


will  be- 


3-34 


20.66.     If  the  increase  in  volume  had  taken. 


place  without  any  alteration  in  the  geometric  form  of  the 
body,  it  should  be  equal  to  the  cube  of  the  increase  in  height ; 
this,  however,  is  3.37^  =  38.27,  a  number  well-nigh  twice 
as  large  as  the  actual  increase. 

But  in  addition  to  these  changes,  which  are  largely  de- 
pendent upon  differences  in  the  supply  of  nutrition,  there 
are  others  associated  with  alterations  in  the  general  metab- 
olism of  the  body.  Up  to  adult  life  the  constructive  metab- 
olism or  anabolism  is  in  excess  of  the  destructive  metabolism 
or  katabolism,  but  the  amount  of  the  excess  is  much  greater 
during  the  earlier  periods  of  development  and  gradually 
diminishes  as  the  adult  condition  is  approached.  That  this 
is  true  during  intrauterine  life  is  shown  by  the  following 
figures,  compiled  by  Donaldson : 


Age  in  Weeks. 

Weight  in  Orams. 

Age  in  Weeks. 

24 
28 
32 
36 

40  (birth) 

Weight  in  Grams. 

0    (ovum) 

4 

8 
12 
16 
20 

0.0006 

4 

20 

120 

285 

635 
1220 
1700 
2240 
3250 

From  this  table  it  may  be  seen  that  the  embryo  of  eight 
weeks  is  six  thousand  six  hundred  and  sixty-seven  times 
as  heavy  as  the  ovum  from  which  it  started,  and  if  the  in- 
crease of  growth  for  each  of  the  succeeding  periods  of  four 
weeks  be  represented  as  percentages,  it  will  be  seen  that  the 
44 


5o6 


POST-NATAL    DEVELOPMENT. 


rate  of  increase  undergoes  a  rapid  diminution  after  the 
sixteenth  week,  and  from  that  on  diminishes  graduaUy  but 
less  rapidly,  the  figures  being  as  follows : 


Periods  of  Weeks 

Percentage  Increase. 

400     • 
500 

137 
123 

Periods  of  Weeks 

Percentage  Increase. 

8-12 
I2-I6 
16-20 
20-24 

24-28 
28-32 
32-36 
36-40 

92 

39 

32 

45 

That  the  same  is  true  in  a  general  way  of  the  growth 
after  birth  may  be  seen  from  the  following  table,  repre- 
senting the  average  weight  of  the  body  in  English  males 
at  different  years  from  birth  up  to  twenty-three  (Roberts), 
and  also  the  percentage  rate  of  increase. 


Year 

Number  of 

Weight  in 

Percentage 

Cases. 

Kilograms. 

Increase. 

0 

451 

3-2 

I 

(10.8) 

(238) 

2 

2 

14.7'^ 

(36)* 

3 

41 

15-4 

4.8* 

4 

102 

16.9 

9-7 

5 

193 

I8.I 

7-1 

6 

224 

20.1 

II 

7 

246 

22.6 

12.4 

8 

820 

24.9 

10.2 

9 

1425 

27.4 

ID 

10 

1464 

30.6 

"•5 

11 

1599 

32.6 

6.5 

12 

1786 

34-9 

7 

13 

2443 

37-6 

7-7 

14 

2952 

41.7 

10.9 

15 

3II8 

46.6 

II  7 

16 

2235 

53-9 

15-7 

17 

2496 

59-3 

10 

18 

2150 

62.2 

4.9 

19 

1438 

63-4 

1-9 

20 

851 

64.9 

2.5 

21 

738 

65-7 

1.2 

22 

542 

67.0 

1.9 

23 

551 

67.0 

0 

*  From  a  comparison  with  other  similar  tables  there  is  little  doubt 
but  that  the  weight  given  above  for  the  second  year  is  too  high  to  be 
accepted  as  a  good  average.  Consequently  the  percentage  increase  for 
the  second  year  is  too  high  and  that  for  the  third  year  too  low. 


POST-NATAL    DEVELOPMENT. 


507 


Certain  interesting  peculiarities  in  post-natal  growth  be- 
come apparent  from  an  examination  of  this  table.  For 
while  there  is  a  general  diminution  in  the  rate  of  growth, 


I 

Age 

i         P          r,         4.         f,         6         7         a         9         10       n        12        13       14.        IS       16       17       .V         1 

LbsK 

■" 

^ 

- 

N 

\ 

. 

., 

'^ 

!■'/ 

_4 

._ 

-  IZ 

- 

', 

^/ 

', 

^ 

"  w 

'l 

'!<• 

^ 

\ 

■'1 

/ 

f 

\ 

•    8 

^ 

,'' 

^^ 

' 

\ 

\ 

\ 

/ 

■A 

. 

/ 

/ 

^ ' 

s^ 

s 

', 

^ 

_ 

f< 

J*. 

\ 

^/ 

/ 

y 

^s 

s.' 

'> 

— 

< 

' 

h 

-•' 

1    r 

" 

II 


Age 

t       2       3       4-       5        6       7       a        9      JO      11      12      13      1*     15     J 

j5      17     18 

- 

Lbs  14- 

"  12 

\ 

"  10 

V 

^ 

i»> 

'ff 

\ 

-  8 

'f 

•'J 

1 

•^i 

fl 

s 

s 

\i 

••  6 

-^ 

■A 

^^ 

^ 

', 

1 

"   f 

\ 

^ 

N 

>f 

Ij 

..1 

■> 

^ 

vl 

J 

V 

"   2 

-- 

._ 

..- 

•' 

^ 

!5h 

^J 

V 

* 

Fig.  274. — Curves  Showing  the  Annual  Increase  in  Weight  in  (I) 
Boys  and  (II)   Girls. 

The  faint  line  represents  the  curve  from  British  statistics,  the  dotted 
line  that  from  American  (Bowditch),  and  the  heavy  line  the  aver- 
age of  the  two.  Before  the  sixth  year  the  data  are  unreliable. — 
{^Stephenson?) 

yet   there   are   marked   irregularities,    the   most   noticeable 
being  (i)  a  rather  marked  diminution  during  the  eleventh 

It  may  be  mentioned  that  the  weights  in  the  original  table  are  ex- 
pressed in  pounds  avoirdupois  and  have  been  here  converted  into 
kilograms,  and  further  the  figures  representing  the  percentage  increase 
have  been  added. 


5o8 


POST-NATAL    DEVELOPMENT. 


and  twelfth  years,  followed  by  (2)  a  rapid  acceleration 
which  reaches  its  maximum  at  about  the  sixteenth  year 
and  then  very  rapidly  diminishes.  These  irregularities  may 
be  more  clearly  seen  from  the  charts  on  page  507,  which  rep- 
resent the  curves  obtained  by  plotting  the  annual  increase 
of  weight  in  boys  (Chart  I)  and  girls  (Chart  II).  The 
diminution  and  acceleration  of  growth  referred  to  above 
are  clearly  observable,  and  it  is  interesting  to  note  that  they 
occur  at  earlier  periods  in  girls  than  in  boys,  the  diminution 
occurring  in  girls  at  the  eighth  and  ninth  years  and  the 
acceleration  reaching  its  maximum  at  the  thirteenth  year. 

Considering,  now,  merely  the  general  diminution  in  the 
rate  of  growth  which  occurs  from  birth  to  adult  life,  it 
becomes  interesting  to  note  to  what  extent  the  organs  which 
are  more  immediately  associated  with  the  metabolic  activi- 
ties of  the  body  underg-o  a  relative  reduction  in  weight. 
The  most  important  of  these  organs  is  undoubtedly  the 
liver,  but  with  it  there  must  also  be  considered  the  thyreoid 
and  thymus  glands,  and  probably  the  suprarenal  bodies.  In 
all  these  organs  there  is  a  marked  diminution  in  size  as  com- 
pared with  the  weight  of  the  body,  as  will  be  seen  from  the 
following  table   (H.  Vierordt),  which  also  includes  data 

ABSOLUTE    WEIGHT    IN    GRAMS. 
New-born  and  Adult. 


Liver. 

Thy- 
reoid. 

Thy- 
mus. 

Suprarenal 
Bodies 

Spleen. 

Heart. 

Kid- 
ney. 

Brain 

Spinal 
Cord. 

141. 7 
1819.0 

4-85 

33-8 

8.15 
26.9 

7.05 

7-4 

10.6 
163.0 

23.6 
300.6 

23-3 
305-9 

381.0 
1430.9 

5-5 

39- 15 

PERCENTAGE    WEIGHT    OF    ENTIRE    BODY. 

New-born  and  Adult. 


Liver. 

Thy. 
reoid. 

Thy- 
mus. 

Suprarenal 
Bodies. 

Spleen 

Heart. 

Kd- 
ney. 

Brain. 

Spinal 
Cord. 

4-57 
2.57 

0.16 
0.05 

0.26 
0.04 

0.23 
O.OI 

0.34 
0.25 

0.76 
0.46 

075 
0.46 

12.29 
2.16 

0.18 
0.06 

POST-NATAL    DEVELOPMENT. 


509 


regarding  other  organs  in  which  a  marked  relative  diminu- 
tion, not  in  all  cases  readily  explainable,  occurs. 

Recent  observations  by  Hammar  render  necessary  some 
modification  of  the  figures  given  for  the  thymus  in  the  above 
table.  He  finds  the  average  weight  of  the  gland  at  birth 
to  be  13.26  grms.,  and  that  the  weight  increases  up  to  puberty, 
averaging  37.52  grms.  between  the  ages  of  11  and  15.  After 
that  period  it  gradually  diminishes,  falling  to  16.27  grms. 
between  36  and  45  and  to  6.0  grms.  between  66  and  75.  Ex- 
pressed in  percentage  of  the  body  weight  this  gives  a  value 
in  the  new-born  of  0.42  and  in  an  individual  of  50  years  of 
0.02,  a  difference  much  more  striking  than  that  shown  in  Vier- 
ordt's  table. 

It  must  be  mentioned,  however,  that  the  gland  is  subject  to 
much  individual  variation,  being  largely  influenced  by  nutritive 
conditions. 

The  remaining  organs,  not  included  in  the  tables  given 
above,  when  compared  with  the  weight  of  the  body,  either 
show  an  increase  or  remain  practically  the  same. 


ABSOLUTE    WEIGHT    IN    GRAMS. 
New-born  and  Adult. 


Skin  and  Subcutan- 
eous Tissues. 

Skeleton. 

Musculature. 

Stomach  and 
Intestines. 

Pancreas. 

Lungs 

611.75 
II765.O 

425-5 

"S7S-0 

776.5 
28732.0 

65 

1364 

3-5 
97-6 

54-1 
994-9 

PERCENTAGE    OF   BODY-WEIGHT. 
New-born  and  Adult. 

Skin  and   Subcutan- 
eous Tissues. 

Skeleton.         Musculature 

Stomach  and 
Intestines. 

Pancreas. 

Lungs. 

1973 

17-77 

13-7              25.05 
17.48            43.40 

2.1 
2.06 

0.  II 
0.15 

1-75 
1.50 

From  this  table  it  will  be  seen  that  the  greatest  increment 
of  weight  is  that  furnished  by  the  muscles,  the  percentage 
weight  of  which  is  one  and  three-fourths  times  as  great  in 
the  adult  as  in  the  child.  The  difference  does  not,  however, 
depend  upon  the  differentiation  of  additional  muscles ;  there 


5IO  POST-NATAL    DEVELOPMENT. 

are  just  as  many  muscles  in  the  new-born  child  as  in  the 
adult,  and  the  increase  is  due  merely  to  an  enlargement  of 
organs  already  present.  The  percentage  weight  of  the 
digestive  tract,  pancreas,  and  lungs  remains  practically 
the  same,  while  in  the  case  of  the  skeleton  there  is  an 
appreciable  increase,  and  in  that  of  the  skin  and  subcuta- 
neous tissue  a  slight  diminution.  The  latter  is  readily 
understood  when  it  is  remembered  that  the  area  of  the  skin, 
granting  that  the  geometric  form  of  the  body  remains  the 
same,  would  increase  as  the  square  of  the  length,  while  the 
mass  of  the  body  would  increase  as  the  cube,  and  hence  in 
comparing  weights  the  skin  might  be  expected  to  show  a 
diminution  even  greater  than  that  shown  in  the  table. 

The  increase  in  the  weight  of  the  skeleton  is  due  to  a 
certain  extent  to  growth,  but  chiefly  to  a  completion  of  the 
ossification  of  the  cartilage  largely  present  at  birth.  A  com- 
parison of  the  weights  of  this  system  of  organs  does  not, 
therefore,  give  evidence  of  the  many  changes  of  form  which 
may  be  perceived  in  it  during  the  period  under  considera- 
tion, and  attention  may  be  drawn  to  some  of  the  more  im- 
portant of  these  changes. 

In  the  spinal  column  one  of  the  most  noticeable  peculiari- 
ties observable  in  the  new-born  child  is  the  absence  of  the 
curves  so  characteristic  of  the  adult.  These  curves  are  due 
partly  to  the  weight  of  the  body,  transmitted  through  the 
spinal  column  to  the  hip-joint  in  the  erect  position,  and 
partly  to  the  action  of  the  muscles,  and  it  is  not  until  the 
erect  position  is  habitually  assumed  and  the  musculature 
gains  in  development  that  the  curvatures  became  pronounced. 
Even  the  curve  of  the  sacrum,  so  marked  in  the  adult,  is  but 
slight  in  the  new-born  child,  as  may  be  seen  from  Fig.  275, 
in  which  the  ventral  surfaces  of  the  first  and  second  sacral 
vertebrae  look  more  ventrally  than  posteriorly,  so  that  there 
is  no  distinct  promontory. 


POST-NATAL    DEVELOPMENT. 


511 


But,  in  addition  to  the  appearance  of  the  curvatures, 
other  changes  also  occur  after  birth,  the  entire  column  be- 
coming much  more  slender  and  the  proportions  of  the  lum- 


FiG.  275. — Longitudinal  Section  through  the  Sacrum  of  a  New- 
born Female  Child. —  (Fehling.) 

bar  and  sacral  vertebrae  becoming  quite  different,  as  may 
be  seen  from  the  following  table  (Aeby)  : 

LENGTHS  OF  THE  VERTEBRAL  REGIONS  EXPRESSED  AS 
PERCENTAGES  OF  THE  ENTIRE  COLUMN. 


Lumbar. 


New-born  child, 
Male    2  years 

"  5  " 
"  II  " 
"     adult 


512  POST-NATAL    DEVELOPMENT, 

The  cervical  region  diminishes  in  length,  while  the  lumbar 
gains,  the  thoracic  remaining  approximately  the  same.  It 
may  be  noticed,  furthermore,  that  the  difference  between 
the  two  variable  regions  is  greater  during  youth  than  in  the 
adult,  a  condition  possibly  associated  with  the  general  more 
rapid  development  of  the  lower  portion  of  the  body  made 
necessary  by  its  imperfect  development  during  fetal  life. 
The  difference  is  due  to  changes  in  the  vertebrae,  the  inter- 
vertebral disks  retaining  approximately  the  same  relative 
thickness  throughout  the  period  under  consideration. 

The  form  of  the  thorax  also  alters,  for  whereas  in  the 
adult  it  is  barrel-shaped,  narrower  at  both  top  and  bottom 
than  in  the  middle,  in  the  new-born  child  it  is  rather  conical, 
the  base  of  the  cone  being  below.  The  difference  depends 
upon  slight  differences  in  the  form  and  articulations  of  the 
ribs,  these  being  more  horizontal  in  the  child  and  the  open- 
ing of  the  thorax  directed  more  directly  upward  than  in 
the  adult. 

As  regards  the  skull,  the  processes  of  growth  are  very 
complicated.  Cranium  and  brain  react  on  one  another,  and 
hence,  in  harmony  with  the  relatively  enormous  size  of  the 
brain  at  birth,  the  cranial  cavity  has  a  relatively  greater 
volume  in  the  child  than  in  the  adult.  The  fact  that  the 
entire  roof  and  a  considerable  part  of  the  sides  of  the  skull 
are  formed  of  membrane  bones  which,  at  birth,  are  not  in 
sutural  contact  with  one  another  throughout,  gives  oppor- 
tunity for  considerable  modifications,  and,  furthermore,  the 
base  of  the  skull  at  the  early  stage  still  contains  a  consider- 
able amount  of  unossified  cartilage.  Without  entering  into 
minute  details,  it  may  be  stated  that  the  principal  general 
changes  which  the  skull  undergoes  in  its  post-natal  develop- 
ment are  (i)  a  relative  elongation  of  its  anterior  portion 
and  (2)  an  increase  in  the  relative  height  of  the  maxillae. 

If  a  line  be  drawn  between  the  central  points  of  the  occip- 


-       POST-NATAL    DEVELOPMENT.  513 

ital  condyles,  it  will  divide  the  base  of  the  skull  into  two  por- 
tions, which  in  the  child's  skull  are  ecjual  in  length.  The 
portion  of  the  skull  in  front  of  a  similar  line  in  the  adult 
skull  is  very  much  greater  than  that  which  lies  behind,  the 
proportion  between  the  two  parts  being  5 :  3,  against  3 :  3 
in  the  child  (Froriep).  There  has,  therefore,  been  a  decid- 
edly more  rapid  growth  of  the  anterior  portion  of  the  skull, 
a  growth  which  is  associated  with  a  corresponding  increase 
in  the  dorso-ventral  dimensions  of  the  maxillae.  These 
bones,  indeed,  play  a  very  important  part  in  determining 
the  proportions  of  the  skull  at  different  periods.  They  are 
so  intimately  associated  with  the  cranial  portions  of  the 
skull  that  their  increase  necessitates  a  corresponding  in- 
crease in  the  anterior  part  of  the  cranium,  and  their  increase 
in  this  direction  stands  in  relation  to  the  development  of  the 
teeth,  the  eight  teeth  which  are  developed  in  each  maxilla 
(including  the  premaxilla)  in  the  adult  requiring  a  longer 
bone  than  do  the  five  teeth  of  the  primary  dentition,  these 
again  requiring  a  greater  length  when  completely  developed 
than  they  do  in  their  immature  condition  in  the  new-born 
child. 

But  far  more  striking  than  the  difference  just  described 
is  that  in  the  relative  height  of  the  cranial  and  facial  regions 
(Fig.  276).  It  has  been  estimated  that  the  volumes  of  the 
two  portions  have  a  ratio  of  8 :  i  in  the  new-born  child,  4:  i 
at  five  years  of  age,  and  2:1  in  the  adult  skull  (Froriep), 
and  these  differences  are  due  principally  to  changes  in  the 
vertical  dimensions  of  the  maxillae.  As  with  the  increase 
in  length,  the  increase  now  under  consideration  is,  to  a  cer- 
tain extent  at  least,  associated  with  the  development  of  the 
teeth,  these  structures  calling  into  existence  the  alveolar 
processes  which  are  practically  wanting  in  the  child  at  birth. 
But  a  more  important  factor  is  the  development  of  the  max- 
illary sinuses,  the  practically  solid  bodies  of  the  maxillae 


5H 


POST-NATAL    DEVELOPMENT. 


becoming  transformed  into  hollow  shells.  These  cavities, 
together  with  the  sinuses  of  the  sphenoid  and  frontal  bones, 
which  are  also  post-natal  developments,  seem  to  stand  in 
relation  to  the  increase  in  length  of  the  anterior  portion  of 
the  skull,  serving  to  diminish  the  weight  of  the  portion  of 
the  skull  in  front  of  the  occipital  condyles  and  so  relieving 
the  muscles  of  the  neck  of  a  considerable  strain  to  which 
they  would  otherwise  be  subjected. 

These  changes  in  the  proportions  of  the  skull  have,  of 
course,  much  to  do  with  the  changes  in  the  general  pro- 


FiG.  276. — Skull  of  a  New-born  Child  and  of  an  Adult  Man,  Drawn 
AS  OF  Approximately  the  Same  Size. —  (Hcnke.) 


portions  of  the  face.  But  the  changes  which  take  place 
in  the  mandible  are  also  important  in  this  connection,  and 
are  similar  to  those  of  the  maxillae  in  being  associated  with 
the  development  of  the  teeth.  In  the  new-born  child  the 
horizontal  ramus  is  proportionately  shorter  than  in  the 
adult,  while  the  vertical  ramus  is  very  short  and  joins  the 
horizontal  one  at  an  obtuse  angle.  The  development  of 
the  teeth  of  the  primary  dentition,  and  later  of  the  three 
molars,  necessitates  an  elongation  of  the  horizontal  ramus 
equivalent  to  that  occurring  in  the  maxillae,  and,  at  the  same 
time,  tlie  separation  of  the  alveolar  borders  of  the  two  bones 
requires  an  elongation  of  the  vertical  ramus  if  the  condyle 


POST-NATAL    DEVELOPMENT. 


515 


is  to  preserve  its  contact  with  the  mandibular  fossa,  and  this, 
again,  demands  a  diminution  of  the  angle  at  which  the 
rami  join  if  the  teeth  of  the  two  jaws  are  to  be  in  proper 
apposition. 

In  the  bones  of  the  appendicular  skeleton  secondary  epi- 
physial centers  play  an  important  part  in  the  ossification, 
and  in  few  are  these  centers  developed  prior  to  birth,  while 
the  union  of  the  epiphyses  to  the  main  portions  of  the  bones 
takes  place  only  toward  maturity.  The  dates  at  which  the 
various  primary  and  secondary  centers  appear,  and  the  time 
at  which  they  unite,  may  be  seen  from  the  following  table : 


UPPER   EXTREMITY. 


Hone. 

Appearance  of 

Appearance  of  Secondary 

Fusion  of 

Primary  Center. 

Centers 

Centers. 

Clavicle 

6th  week. 

(At  Sternal  end)  17th  year. 

20th  year. 

Scapula. 

Body 

8th  zveek.    | 

2  acromial  15th  year. 

2  on  vertical  border  i6th  year. 

\  20th  year. 

Coracoid  ... 

1st  year. 

15  th  year. 

' 

Head  1st  year. 

) 

Great  tuberosity  3d  year. 

\  20th  year. 

Lesser  tuberosity  5th  year. 

J 

Humerus 

•jlh  week.   ' 

Inner  condyle  5th  year. 
Capitellum  3d  year. 

) 

Trochlea  loth  year. 

\  17th  year. 

V 

Outer  condyle  14th  year. 

] 

Ulna 

•/th  week. 

Olecranon  loth  year. 
Distal  epiphysis  4th  year. 

1 6th  year. 
i8th  year. 

Radius 

yth  week. 

Proximal  epiphysis  5lh  year. 
Distal  epiphysis  2d  year. 

17th  year. 
20th  year. 

Capitatum 

1st  year. 

Hamatum 

2d  year. 

Triquetrum. ... 

3d  year. 

Lunatum 

4th  year. 

Multangulum 

Sth    year. 

majus 

Navicular 

6th  year. 

Multangulum 

8th  year. 

minus 

Pisiform 

1 2th  year. 

Metacarpals  .. 

gth  week. 

3d  year. 

20th  year. 

Phalanges 

glh-iith 

3d-5th  years. 

I7th-i8th 

lueek. 

year. 

The  dates  in  italics  are  before  birth. 


516 


POST-NATAL    DEVELOPMENT, 


LOWER    EXTREMITY. 


Appearance  of 

Appearance  of  Secondary 

Fusion  of  Cen- 

Primary Center. 

Centers. 

ters. 

Ilium 

gth  week. 

Crest  15th  year. 

>| 

Anterior  inferior  spine  15th  year. 

[-  22d  year. 

Ischium 

^th  7>ionth, 

Tuberosity  15th  year. 

Pubis 

^th  month. 

Patella 

Cartilage  appears  at  4th  month,  ossification  in  3d 

year. 

" 

Head  1st  year. 

20th  year. 

Great  trochanter  4th  year. 

19th  year. 

Femur 

yth  week. 

Lesser  trochanter  I3lh-I4th  year. 
Condyle  gth  month. 

1 8th  year. 
2ist  year. 

Tibia 

yth  week.    \ 

Head  end  of  gth  month. 
Distal  end  2d  year. 

2ist-25thyear. 

l8th  year. 

Fibula 

8th  zveek.    \ 

Upper  epiphysis  5th  year. 

2 1st  year. 

Lower  epiphysis  2d  year. 

20th  year. 

Talus 

jth  month. 
6th  month. 

loth  year. 

Calcaneus 

i6th  year. 

Cuboid 

A  few  days 
after  birth. 

Navicular 

4th  year. 

Cuneiforms.... 

1st  year. 

Metatarsals  ... 

gth  week. 

3d  year. 

20th  year. 

Phalanges 

gth-i2tk 
zveek. 

4th-Sth  years. 

The  dates  in  italics  are  before  birth. 


So  far  as  actual  changes  in  the  form  of  the  appendicular 
bones  are  concerned,  these  are  most  marked  in  the  case  of 
the  lower  limb.  The  ossa  innominata  alter  somewhat  in 
their  proportions  after  birth,  a  fact  which  may  conveniently 
be  demonstrated  by  considering  the  changes  which  occur  in 
the  proportions  of  the  pelvic  diameters,  although  it  must 
be  remembered  that  these  diameters  are  greatly  influenced 
by  the  development  of  the  sacral  curve.  Taking  the  conju- 
gate diameter  of  the  pelvic  brim  as  a  unit  for  comparison, 
the  antero-posterior  (dorso-ventral)  and  transverse  diame- 
ters of  the  child  and  adult  have  the  proportions  shown  in 
the  table  on  the  opposite  page  (Fehling). 

It  will  be  seen  from  this  that  the  general  form  of  the 
pelvis  in  the  new-born  child  is  that  of  a  cone,  gradually 
diminishing  in  diameter  from  the  brim  to  the  outlet,  a  con- 
dition very  different  from  what  obtains  in  the  adult.     Fur- 


POST-NATAL    DEVELOPMENT. 


517 


Diameter. 


Conjugata  vera,  . 

•^  (   Transverse, 

>.  f  Antero-po^terior, 

'O  (   Transverse, 

„•  r  Anteroposterior, 
O  (  Transverse, 


New-born 
Female. 

Adult 
Female. 

New-born 
Male. 

1. 00 

1. 00 

1. 00 

1. 19 

1.292 

1.20 

0.96 

1. 19 

0.91 

1. 01 

1. 151 

0.99 

0.91 

1.05 

0.78 

0.83 

IIS4 

0.84 

Adu't 
Male. 


1. 00 
1.294 
1. 18 
1. 14 
1.07 
II53 


thermore,  it  is  interesting"  to  .note  that  sexual  differences  in 
the  form  of  the  pelvis  are  clearly  distinguishable  at  birth; 
indeed,  according  to  Fehling's  observations,  they  become 
noticeable  during  the  fourth  month  of  intrauterine  develop- 
ment. 

The  upper  epiphysis  of  the  femur  is  entirely  unossified 
at  birth  and  consists  of  a  cartilaginous  mass,  much  broader 
than  the  rather  slender  shaft  and  possessing  a  deep  notch 
upon  its  upper  surface  (Fig.  277).  This  notch  marks  off 
the  great  trochanter  from  the  head  of  the  bone,  and  at  this 
stage  of  development  there  is  no  neck,  the  head  being  prac- 
tically sessile.  As  development  proceeds  the  inner  upper 
portion  of  the  shaft  grows  more  rapidly  than  the  outer  por- 
tion, carrying  the  head  away  from  the  great  trochanter  and 
forming  the  neck  of  the  bone.  The  acetabulum  is  shallower 
at  birth  than  in  the  adult  and  cannot  contain  more  than  half 
the  head  of  the  femur ;  consequently  the  articular  portion  of 
the  head  is  much  less  extensive  than  in  the  adult. 

It  is  a  well-known  fact  that  the  new-born  child  habitually 
holds  the  feet  with  the  soles  directed  toward  one  another, 
a  position  only  reached  in  the  adult  with  some  difficulty,  and 
associated  with  this  supination  or  inversion  there  is  a  pro- 
nounced extension  of  the  foot  (i.  <?.,  flexion  upon  the  leg  as 
usually  understood;  see  p.  91),  it  being  difficult  to  flex 
the  child's  foot  beyond  a  line  at  right  angles  with  the  axis 


5i8 


POST-NATAL    DEVELOPMENT, 


of  the  leg.  These  conditions  are  due  apparently  to  the  ex- 
tensor and  tibialis  muscles  being  relatively  shorter  and  the 
opposing  muscles  relatively  longer  than  in  the  adult,  and 
with  the  elongation  or  shortening,  as  the  case  may  be,  of 
the  muscles  on  the  assumption  of  the  erect  position,  the 
bones  in  the  neighborhood  of  the  ankle-joint  come  into  new 
relations  to  one  another,  the  result  being  a  modification  of 
the  form  of  the  articular  surfaces,  especially  of  the  talus 


Fig.  277. — Longitudinal  Sections  of  the  Head  of  the  Femur  of  (A) 

New-born  Child  and   (B)   a  Later  Stage  of  Development. 
ep,  Epiphysial  center  for  the  head;  h,  head;  t,  trochanter. —  (Henke.) 

(astragalus).  In  the  child  the  articular  cartilage  of  the 
trochlear  surface  of  this  bone  is  continued  onward  to  a  con- 
siderable extent  upon  the  neck  of  the  bone,  which  comes  into 
contact  with  the  tibia  in  the  extreme  extension  possible  in 
the  child.  In  the  adult,  however,  such  extreme  extension 
being  impossible,  the  cartilage  upon  the  neck  gradually  dis- 
appears. The  supination  in  the  child  brings  the  talus  in 
close  contact  with  the  inner  surface  of  the  calcaneus  and 
with  the  sustentaculum  tali ;  with  the  alteration  of  position 
a  growth  of  these  portions  of  the  calcaneus  occurs,  the  sus- 


POST-NATAL    DEVELOPMENT.  5I9 

tentaculum  becoming  higher  and  broader,  and  so  becoming 
an  obstacle  in  the  way  of  supination  in  the  adult.  At  the 
same  time  a  greater  extent  of  the  outer  surface  of  the  talus 
comes  into  contact  with  the  lateral  malleolus,  with  the  result 
that  the  articular  surface  is  considerably  increased  on  that 
portion  of  the  bone.  Marked  changes  in  the  form  of  the 
talo-navicular  articulation  also  occur,  but  their  consideration 
would  lead  somewhat  further  than  seems  desirable. 

LITERATURE. 

C.  Aeby  :  "  Die  Altersverschiedenheiten  der  menschlichen  Wirbelsaule." 

Archiv  fiir  Anat.  und  Physiol.,  Anat.  Abth.,  1879. 
W.  Camerer  :  "  Untersuchungen  iiber  Massenwachsthum  und  Langen- 

wachsthum   der    Kinder,"    Jahrbuch   fiir   Kinderheilkunde,    xxxvi, 

1893. 
H.  H.  Donaldson  :  "  The  Growth  of  the  Brain,"  London,  1895. 
H.  Fehling  :  "  Die  Form  des  Beckens  beim  Fotus  und  Neugeborenen 

und  ihre  Beziehung  zu  der  beim  Erwachsenen,"  Archiv  fiir  Gyn'd- 

kol.,  X,  1876. 
J.  A.  Hammar:  "Ueber  Gewicht,  Involution  und  Persistenz  der  Thy- 
mus   im    Postfotalleben    des    Menschen,"    Archiv    fiir    Anat.    und 

Phys.,  Anat.  Abth.,  Supplement,  1906. 
W.    Henke:    "  Anatomie   des    Kindersalters,"   Handbuch   der  Kinder- 

krankheiten   (Gerhardt),  Tubingen,  1881. 
C.  Hennig  :  "  Das  kindliche  Becken,"  Archiv  fiir  Anat.  und  Physiol, 

Anat.  Abth.,   1880. 
C.  HuTER :  "  Anatomische  Studien  an  den  Extremitatengelenken  Neuge- 

borener  und  Erwachsener,"  Archiv  fiir  patholog.  Anat.  und  Physiol, 

XXV,  1862. 
W.  Stephenson  :  "  On  the  Relation  of  Weight  to  Height  and  the  Rate 

of  Growth  in  Man,"  The  Lancet,  11,  1888. 
R.  Thoma:   "Untersuchungen  iiber  die  Grosse  und  das   Gewicht  der 

anatomischen    Bestandtheile   des   menschlichen   Korpers,"   Leipzig, 

1882. 
H.  Vierordt:   "Anatomische,   Physiologische  und   Physikalische  Daten 

und  Tabellen,"  Jena,  1893. 
H.  Welcker  :  "  Untersuchungen  iiber  Wachsthum  und  Bau  des  mensch- 
lichen  Schadels,"  Leipzig,   1862. 


NDEX. 


A 

After-birthj    144 
After-brain,  411 
Agger  Nasi,    186 
Allantois,    114,    119 
Alveolo-lingual  glands,   311 

groove,   306 
Amitotic  division,  7 
Amnion,    115 
Amniotic  cavity,  57 
Amphiarthrosis,    199 
Amphiaster,   6 
Annulus  of  Vieussens,   246 
Anterior  commissure,  431 
Anthelix,  477 
Antitragus,    477 
Anus,   299 
Aortic  arches,  256 

septum,  249 
Archenteron,   49,   296 
Archoplasm   sphere,   4 
Arcuate  fibers,   415 
Areas  of  Langerhans,   332 
Arrectores  pilorum,   154 
Arteries,    253 

allantoidean,    255 

antferior    tibial,    267 

aorta,    257 

branchial,    255 

carotid,    256 

centralis   retinae,    491 

caeliac,    259 

common  iliac,  258 

epigastric,  262 

external   iliac,   267 
maxillary,  256 

femoral,   267,    269 

hyaloid,  480 
.  hypogastric,    259,   261 

inferior  mesenteric,  259 

innominate,  257 

intercostal,  258 

internal  mammary,  262 
maxillary,   256 
spermatic,  259 

45 


Arteries — continued 

interosseous,   265 

lingual,  256 

lumbar,  258 

median,  265 

middle   sacral,    259 

omphalo-mesenteric,    119, 

peroneal,    269 

popliteal,    267 

posterior  tibial,  269 

pulmonary,  256 
'  radial,    267 

renal,  259 

saphenous,  267 

sciatic,   267 

subclavian,  268 

superficial  radial,   265 

superior   intercostal,    262 
mesenteric,  259 
vesical,   261 

temporal,   256 

ulnar,  265 

umbilical,  122,  255,  260 

vertebral,   262 

vitelline,    235 
Articular  capsule,   199 
Ary-epiglottic   folds,   356 
Arytenoid  cartilages,  358 
Aster,  4 
Atresia  of  duodenum,  325 

of  pupil,  484 
Atrial  septum,  246 
Atrio-ventricular   valves,    251 
Auerbach,  plexus  of,  448 
Auricle,  476 
Axis   cylinder,   402 

B 
Bartholin,  glands  of,  384 
Belly-stalk,   70,   120 
Bile  capillaries,  329 
Bladder,   382 
Blastoderm,  44 
Blastopore,  49,  60 
Blastula,  41 


235 


521 


522 


INDEX. 


Blood,  237 

islands,  234 
vessels,  233 
Body    cavity,    50 
Bone,  development  of,  161 

growth  of,   164 
Bone-marrow,   163 
Bones : 

atlas,  171,  174 
axis,   171,  174 
carpal,  19s,  515 
clavicle, .193,  515 
coccyx,   175 
conchse,  186 
epistropheus,   171,   174 
ethmoid,    184 
femur,  197,  516,  517 
fibula,  197,  516 
frontal,   188 
humerus,   194,   515 
hyoid,  191 
ilium,  196,  515 
incus,    190,  471 
innominate,   196,   516 
interparietal,    182 
ischium,   197,  516 
lachrymal,    188 
malleus,   190,  471 
mandible,  190 
maxilla,    189 
metacarpal,  196,  515 
metatarsal,   198,  516 
nasal,    188 
occipital,  178,  182 
palatine,    189 
parietal,   188 
patella,    197,   516 

periotic,  178,  186 

phalanges,    196,    198,   515,   516 

premaxilla,    190 

pubis,   197,   516 

radius,   195,   515 

ribs,    172 

sacrum,   174 

scapula,    194,   515 

sphenoid,    183 

stapes,   471 

sternum,    175 

suprasternal,    175 

tarsal,    198,   516 

temporal,   187 

tibia,    197,    516 

turbinated,   186 

ulna,  19s,  515 

vertebrte,  168,  173,  511 


Bones — continued 

vomer,    185 

zygomatic,    188 
Brachia  conjunctiva,  419 
Brain,  409 
Branchial  arches,  •](>,  85 

clefts,   "]() 

epithelial  bodies,  313 

fistula,    'J^ 
Branchiomeres,   107 
Bronchi,  354 

Bulbo-urethral  glands,   384 
Bulbo-vestibular  glands,  384 
Burdach,  fasciculus  of,  409 

C 
Cjecum,  323 
Calcar,  429 

Canal  of  Cloquet,  496 
of   Gaertner,   380 
of  Nuck,  388 
of  Petit,  496 
Capillaries,  235 
Cartilages  of  Santorini,  358 

of  Wrisberg,   358 
Caruncula  lacrimalis,  501 
Caul,  118 
Cell,  I 

Cell-theory,   i 
Centrosome,  4 
Cerebellum,   417 
Cerebral  aqueduct,  420 
cortex,   434 
hemispheres,  425 
convolutions,  428 
peduncles,   419 
Cheek  groove,  309 
Chin  ridge,  89 
Chondrocranium,   178,   181 
Chorda  canal,  60 
dorsalis,    100 
endoderm,   100 
Chorioid  coat,  496 

plexus,  413,  423,  427 
Chorioidal  fissure  of  brain,  426 

of  eye,  479,  485 
Chorion,    69,    125 
Chorionic  villi,    128 
Chromaffine  organs,   392 
Chromatin,  4 
Chromosomes,   5 
Ciliary  body,  486 
ganglion,  449 
muscle,  498 
Cleft  palate,  300 
sternum,  177 


INDEX. 


523 


Clitoris,   385 
Cloaca,  296,  382 
Cloacal  membrane,  297 
Cloquet,   canal   of,   496 
Coccygeal  ganglion,  292 
Cochlea,   368 
Coelom,   50 

Collateral   eminence,   431 
Coloboma,  485 
Colon,   322 
Conjunctiva,  498 
Connective  tissues,   160 
Cornea,  498 

Corniculate  cartilages,  358 
Coronary  sinus,  245 
Corpora  mamillaria,  423 

quadrigemina,  420 
Corpus  albicans,  24 

callosum,  431 

luteum,  23 

striatum,  426 
Corti,  spiral   organ  of,   467 
Cowper,  glands  of,  384 
Cranial  nerves,  436 

sinuses,  270 
Cricoid  cartilage,   358 
Cuneiform   cartilages,   358 
Cutis  plate,    106 
Cytoplasm,    4 

D 

Darwin's  tubercle,  477 
Decidua  basalis,  134,   138 

capsularis,  128,  136 

reflexa,  128 

serotina,  134,  138 

vera,  134,  135 
Decidual  cells,  136,  144 
Dendrites,  402 
Dental  groove,  301 

shelf,  301 
Dentate  gyrus,   429 
Dermatome,    106 
Descent  of  ovary,  387 

of  testis,   388 
Diaphragm,    339 
Diarthrosis,   199 
Diencephalon,  421 
Discus  proligerus,  18 
Double   monsters,    48 
Duck  of  Santorini,   332 

of   Wrisberg,    332 
Ductus  arteriosus,  257 

Botalli,   257 

cochlearis,   464 


Ductus  Cuvieri,  2^2 
ejaculatorius,  378 
endolymphaticus,   462 
reuniens,   464 
venosus,  276 

Duodenum,   321,    322,   324 

E 
Ear,   461 

Ebner,   glands   of,   460 
Ectoderm,   49 
Embryo,  age  of,  91 

external  form,  67 

growth   of,   506 
Embryonic  disc,  57 
Enamel   organ,    301 
Enchylema,  4 
Endoderm,  49 
Enveloping  layer,  44 
Ependymal  cells,  401 
Epiblast,   49 

Epibranchial   ganglia,   445 
Epidermis,    147 
Epididymis,    377 
Epiglottis,    356 
Epiphyses,    164 
Epiphysis   cerebri,   421 
Epiploic   foramen,    344 
Episternal   cartilages,    175 
Epitrichium,    147 
Eponychium,    152 
Epoophoron,  378 
Erythroblasts,  239 
Erythrocytes,    234 
Erythroplastids,  239 
Eustachian  tube,   311,  471 

valve,  247 
Extrauterine  pregnancy,  21 
Eye,   477 
Eyelids,   498 


Fallopian"  tubes,  350 
Fasciculus   communis,  441 

of   Burdach,    409 

of  Goll,  409 

solitarius,   441 
Fenestra  cochlete,  470 

ovalis,    470 

rotunda,  470 

vestibuli,   470 
Fertilization   of  ovum,    31 
Fetal   circulation,  283 
Fifth   ventricle,   432 
Filum   terminale,   407 


"524 


INDEX. 


Foliate   papillse,   461 

Fontana,   spaces  of,  498 

Foramen    caecum,    306,    313 
of   Munro,   426 
of   Winslow,    344 
ovale,    246 

Fore-brain,   410 

Formatio   reticularis,  415 

Fornix,  432 

Frontal   sinuses,    186 

Furcula,    3 1 2 


Gaertner,  canals  of,  380 
Gall   bladder,    325 
Ganglionated  cord,  451 
Gastral  mesoderm,  52,  62 
Gastrula,  49,  63 
Geniculate  bodies,   423 
Genital   folds,   385 

ridge,  369 

swelling,    385 

tubercle,   385 
Germ  cells,  8 

layers,  49 

plasm,  8 
Giraldes,   organ   of,    377 
Glands   of   Bartholin,    384 

of   Cowper,    384 

of  Ebner,  460 

of  Moll,   500 
Goll,  fasciculus  of,  409 
Graafian  follicle,  17 
Great  omentum,   344 
Groove  of  Rosenmiiller,   313 
Gubernaculum   testis,    372 
Gynaecomastia,    159 

H 

Haematopoietic  organs,  237 

HEemolymph   nodes,   290 

Hairs,    152 

Hare   lip,   89,    190 

Head  cavities,   104 

process,    59 
heart,  241 
Helix,  477 
Hensen's    node,    59 
Hermaphroditism,    387 
Hind-brain,   410 
Hippocampus,   429 
Hyaloid  canal,  496 
Hydatid  of  Morgagni,  378 

stalked,  380 


Hyperthelia,   158 
Hypertrichosis,   155 
Hypoblast,  49 
Hypochordal  bar,   169 
Hypophysis,  424 
Hypospadias,   386 
Hypothalamic  region,   423 
Hymen,   380 


Implantation  of  ovum,    125 

Infundibulum,    425 

Inguinal  canal,   390 

Inner  cell  mass,  47 

Insula,  430 

Interarticular   cartilages,    200 

Intercarotid  ganglion,   395 

Intermediate   cell   mass,    104 

Interrenal  organs,  392 

Interventricular  foramen,  426 

Intervertebral    fibro-cartilage,    176 

Intestine,   320 

Iris,  486 

Isthmus   cerebri,   419 


Jacobson,  organ  of,  459 
Joints,   199 

K 
Karyokinesis,  7 
Karyoplasm,   4 
Kidney   (see  Metanephros),  366 


Labia  majora,   386 
minora,   385 

Lachrymal  gland,  500 

Lamina  terminalis,  424 

Langerhans,  areas  of,  332 

Langhans  cells,   129 

Lanugo,    154 

Larynx,  356 

Lateral  thyreoids,  314 

Lens,   480 

Lesser  omentum,   344 

Leukocytes,   240 

Ligaments  : 

broad,  of  ovary,  371 
coraco-humeral,    227 
coronary,  of  liver,   341 
costo-vertebral,    170 
falciform,  of  liver,  341 
fibular  lateral,  of  knee.  209 
flavan,    170 


INDEX. 


525 


Ligaments — continued 
inguinal,    388 
interspinous,    170 
intervertebral,    170 
of  the  ovary,  2)7^ 
pectinatum  iridis,  498 
round,  of  liver,  285 
sacro-tuberous,   209 
spheno-mandibular,    191 
supraspinous,    170 
teres,  of  ovary,  372 

Limbs,   89 

Lip-ridge,    89 

Lips,  299 

Liver,    325 

Lungs,  352 

Luschka's  ganglion,  292 

Lymphatics,   285 

Lymph  hearts,  285 
nodes,  289 

Lymphocytes,  239,   285 

M 
Mammary  gland,   155 
Mandibular  process,   82 
Mastoid  cells,  474 
Maturation  of  ovum,  27 
Maxillary  process,   82 

sinus,    186 
Meckel's  cartilage,    180,    190 

diverticulum,    119,   323 
Mediastina,   342 
Medulla   oblongata,   410 
Medullary  canal,  99 

folds,    71,   96 

groove,    96 

sheath,  405 
Megacaryocytes,   240 
Meibomian  glands,  500 
Meissner,  plexus   of,  453 
Membrana  pupillaris,   484 

reuniens,    107 

tectoria,  468 
Membrane   bone,    162 
Menstruation,    21 
Mesencephalon,  420 
Mesenchyme,  65 
Mesenteriole,    348 
Mesentery,  343 
Mesocardium,   335 
Mesocolon,    347 
Mesoderm,   50 

Mesodermic   somites,    -jz,   103 
Mesogastrium,    344 
Mesonephros,  363 


Mesorchium,  371,  390 
Mesovarium,   371 
Mesothelium,    65 
Metamere,    108 
Metanephros,   366 
Metencephalon,    416 
Mid-brain,   410 
Middle  ear,  470 
Milk   ridge,    156 
Mitosis,  7 

Moll,  glands  of,   500 
Monro,  foramen  of,  426 
Montgomery's  glands,   158 
Morgagni,   hydatid   of,   378 
Morula,  44 
Mouth  cavity,  299 
Miillerian  duct,  369 
Muscle  plates,   106 
Muscles : 

biceps  femoris,   226 

branchiomeric,  214 

chondroglossus,  218 

coccygeus,   212 

constrictor  of  pharynx,  218 

cranial,   214 

curvator  coccygis,  214 

depressors  of  hyoid,  211 

digastric,  216 

dilatator  iridis,  487 

dorsal,   210 

eye,   215 

erector  spinje,  208 

facial,  216 

gastrocnemius,   225,  229 

geniohyoid,    211 

genioglossus,   211 

glosso-palatinus,    218 

hyoglossus,  212 

hyposkeletal,  212 

laryngeal,    218 

latissimus  dorsi,  208 

levator  ani,  212 

limb,   220 

longus   capitis,   211 

colli,  211 
lumbrical,   229 
masseter,   216 
mylohyoid,   216 
obliqui   abdominis,   212 
occipito-frontalis,  208,  216 
omohyoid,  207 
pectorals,    226,   227 
perineal,    214 
peroneus  longus,  209 
platysma,   216 


526 


INDEX, 


Muscles — continued 

psoas,  211 

pterygoids,   216 

pyramidalis,  210 

rectus  abdominis,  208,  210 

scaleni,   212 

serrati  posteriores,  208 

serratus  anterior,   208 

skeletal,  206 

soleus,  225,  229 

sphincter  ani,  214 
cloacae,  214 
iridis,   487 

stapedius,  216,  473 

sternohyoid,  207 

sternomastoid,    207,    212,    2ii 

stylohyoid,   216 

stylopharyngeus,   216 

temporal,  216 

tensor  tympani,  216,  471 
veli  palati,  216 

transversus   abdominis,   212 
thoracis,  208 

trapezius,  207,  212,  218 
Muscle   tissue,    203 
Myelencephalon,  413 
Myelin,  405 
Myoblasts,  204 
Myotome,   106,  206 

N 
Nails,    150 
Nasal  fossae,  81 
process,  88 
Naso-lachrymal  duct,  500 
Nephrostome,  362 
Nephrotome,    106 
Nerve   components,   440 

roots,    402 
Nerves  : 

auditory,   443 
cranial,  436 
hypoglossal,   439 
olfactory,   457 
optic,  491 
recurrent,  359 
spinal,  434 

accessory,   444 
splanchnic,  451 
Nerve  tissue,  400 
Neural  crest,  403 
Neurenteric  canal,  60,  71,  96 
Neuroblasts,  401 
Neuroglia  cells,  401 
Neuromeres,  446 


Neurone  theory,  405 
Non-sexual  reproduction,  9 
Nuck,   canal   of,   388 
Nucleoli,   4 
Nucleus,   4 

O 
CEsophagus,  318 
CEstrus,  23 
Odontoblasts,   304 
Olfactory  lobes,  433 

organ,  457 
Olivary  body,   415 
Omentum,  344 
Oocyte,  2.^ 
Optic  cup,  479,  485 

recess,  424 
Oral  fossa,  74,  296 
Organ  of  Giraldes,  377 
of  Jacobson,  459 
of  Rosenmiiller,   378 
Organs,    3 

of  taste,  460 
of  Zuckerkandl,  398 
Osteoblasts,    162 
Osteoclasts,    165 
Otocyst,  461 
Otic   ganglion,   453 
Ovary,  375 

descent  of,  388 
Ovulation,   20 
Ovum,  17 

implantation   of,    125 
fertilization  of,   31 
maturation  of,  2.-] 
segmentation  of,  39 


Palate,  299 
Pancreas,   331 
Paradidymis,  377 
Paraphysis,   422 
Parathymus,  317 
Parathyreoid  bodies,   31 S 
Paroophoron,   378 
Parotid  gland,  310 
Parovarium,    378 
Parthenogenesis,  9 
Penis,   386 

Pericardial    cavity,    338 
Perineal  body,  384 
Perionyx,   152 
Periosteum,   162 
Periotic  capsule,  178,  186 
Peritoneum,  343 
Petit,  canal  of,  496 


INDEX. 


527 


Pfliiger's  cords,  375 
Pharyngeal  bursa,  312 

membrane,   296 

tonsil,  312 
Pharynx,   311 
Pineal  body,   421 
Pinna,    476 
Pituitary  body,   424 
Placenta,    139 

prsevia,    139 
Pleurae,   342 

Pleuro-peritoneal  cavity,   104,  339 
Plica   semilunaris,    500 
Polar  globules,  28 
Polycaryocytes,  240 
Polymastia,   158 
Polyspermy,    34 
Post-anal  gut,   297 
Post-branchial  bodies,  316 
Post-natal  development,  503 
Precaudal  recess,  298 
Precoracoid,  200 
Prepuce,   386 
Primitive  groove,  59 

streak,  52 
Processus  globularis,  88 
Pronephric   duct,   361 
Pronephros,   361 
Pronuclei,  32 
Prooestrum,   23 
Prostate  gland,  384 
Prostomial  mesoderm,   52,   62 
Protoplasm,   2 
Protovertebrse,    103 

R 

Rathke's  pouch,  301,  424 
Rauber's  covering  layer,  47 
Receptaculum  chyli,  286 
Rectum,  297 
Red  nucleus,  420 
Reduction  of  chromosomes,   15 
Restiform  body,  416 
Rate  cords,  371 

ovarii,  376 

testis,  373 
Retina,  487 
Rhinencephalon,   434 
Rosenmiiller,  groove  of,  313 

organ  of,  378 

S 
Sacculus,   464 
Salivary  glands,   308 
Santorini,  cartilages  of,  358 
duct  of,  332 


Sarcode,  2 

Scala  tympani,  470 

vestibuli,   470 
Sclerotic  coat,  496 
Sclerotome,   106 
Scrotum,   386 
Sebaceous   glands,    154 
Segmentation  of  ovum,  39 
Semicircular  ducts,  462 
Semilunar  valves,  252 
Septum  pellucidum,  423 

primum,  246 

secundupi,  246 

spurium,   245 

transversum,  337,  339,  342 
Sertoli    cell,    13 
Sex  cords,  371 
Sexual  reproduction,  9 
Sinusoid,  236 
Sinus  pocularis,   378 

prEccervicalis,  86 
Situs  inversus  viscerum,  48 
Skin,    147 
Skull,   177,   512 
Socia  parotidis,  310 
Solitary  fasciculus,  414 
Somatic  cells,  8 
Spaces   of  Fontana,  498 
Spermatic  cord,  390 
Spermatid,   14 
Spermatocyte,  14 
Spermatogenesis,    13 
Spermatogonia,   13 
Spermatozoon,    1 1 
Sphenoidal  cells,  186 
Spheno-palatine  ganglion,  453 
Spinal  cord,  406 

nerves,   434 
Spiral  organ  of  Corti,  467 
Spleen,   290 
Stomach,   318 
Sublingual  ganglion,  453 

gland,   310 
Submaxillary  ganglion,   453 

gland,  310 
Substance  islands,  234 
Sudoriparous  glands,   155 
Sulcus  Monroi,  421 
Superfetation,  36 
Suprabranchial  ganglia,  445 
Suprarenal  bodies,  392 

accessory,  395 
Supratonsillar  fossa,  313 
Suture,    199 
Sympathetic  nervous  system,  446 


528 


INDEX. 


Synchondrosis,    199 
Systems,  3 

T 

Tail  filament,   83 
Tarsal  glands,  500 
Taste,  organs  of,  460 
Teeth,  301 
Tegmentum,    419 
Telencephalon,   424 
Testis,  372 

descent  of,  388 
Thalami,   423 
Thebesian   valve,   247 
Thoracic  duct,  286 
Thymus   gland,    316,    508 
Thyreoid  cartilage,  357 

gland,  313 
Thyreo-glossal  duct,   314 
Tissues,   3 
Tongue,  306 
Tonsils,    312 
Touch,   organs   of,  460 
Tragus,  477 
Trophoderm,  57 
Tubas  uterinae,  380 
Tuber  cinereum,  423 
Tuberculum  impar,    306 
Tunica  vaginalis  testis,  389 

vasculosa  lentis,  482 
Tween-brain,   410 
Twin-development,  47 
Tympanic  cavity,  471 
membrane,  474 

U 
Umbilical  cord,   79,   122 
Umbilicus,    T2,,   122 
Urachus,   122,   383 
Ureter,  366 
Urethra,   384 
Urogenital  sinus,   382 
Uterus,   380,   381 

masculinus,  378 
Utriculus,  464 

V 
Vagina,  380 
Vaginal   process,    388 
Vallate  papilhe,  460 
Vas  deferens,   378 
Veins  : 

anterior  cardinal,  269 
tibial,  282 

ascending  lumbar,    281 

azygos,   281 


Veins — continued 

basilic,   282 

cephalic,  282 

emissary,  274 

external  jugular,  273 

hemiazygos,   281 

hepatic,  2yy 

inferior  vena  cava,  279 

innominate,   273 

internal  jugular,  269 

jugulo-cephalic,  282 

limb,  281 

long  saphenous,  282 

omphalo-mesenteric,   235,   274 

portal,  zyy 

posterior  cardinal,   274 

primary  iibular,  282 
ulnar,  282 

pulmonary,  282 

renal,   279 

subcardinal,    277 

superior  vena  cava,  273 

supracardinal,   279 

suprarenal,   279  « 

umbilical,   122 

vitelline,    235 
Velum,  anterior,  419 

interpositum,  422,  426 

marginal,   401 

posterior,   413 
Ventricular  septum,  248 
Vermiform  appendix,  324 
Vernix   caseosa,    117 
Veru  montanum,  380 
Vesicula  seminalis,  378 
Vieussens,  annulus  of,  246 
Villi,  chorionic,  128 

intestinal,  324 
Vitreous  humor,  494 
Vulva,  386 

W 
Wharton's  jelly,   125 
Winslow,   foramen  of,  344 
Wirsung,  duct  of,  332 
Witch  milk,   159 
Wolffian  body,  361,  377 

duct,   361,  Z77 

ridge,  360 
Wrisberg,  cartilage  of,  358 

Y 
Yolk  sac,  68,  114,  118 
stalk,   72,   114 

Z 

Zuckerkandl,  organ  of,  398 


COLUMBIA  UNIVERSITY  LIBRARIES 

This  book  is  due  on  the  date  indicated  below,  or  at  the 
expiration  of  a  definite  period  after  the  date  of  borrowing, 
as  provided  by  the  rules  of  the  Library  or  by  special  ar- 
rangement with  the  Librarian  in  charge. 


DATE  BORROWED 


m 


DATE  BORROWED 


■B 


n5~ 


MAY  15 


1944 


I  7  19$$ 


C2e(1  I  40)  Ml  00 


M22 
1907 


